Aberrant: Atypical, departing from the normal type or structure.

Abiotic: The absence of living organisms.

Abrasion: Wearing away of surfaces by friction.

Absolute pressure: Atmospheric pressure plus gauge pressure.

Absorbed dose: The amount of a chemical that enters the body of an exposed organism.

Absorbed water: Water held mechanically in a soil mass and having physical properties not substantially different from ordinary water at the same temperature and pressure. See adsorbed water.

Absorption: Taking in of fluids or other substances through, or as if through, cells or tissues. The uptake of water or dissolved chemicals by a cell or an organism (as tree roots absorb dissolved nutrients in the soil). Should not be confused with adsorption.

Absorption factor: The fraction of a chemical making contact with an organism that is absorbed by the organism.

Abutment: That part of the valley wall against which the dam is constructed. The part of a dam that contacts the riverbank. A structure that supports the ends of a dam or bridge. An artificial abutment is sometimes constructed, as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. Action or place of abutting; the part of a structure that is the terminal point or receives thrust or pressure. Defined in terms of left and right as looking away from the reservoir, looking downstream (i.e., left abutment, right abutment).

Acceleration: In terms of flow, acceleration is the time rate of change of the velocity vector, either of magitude or direction or both.

Accelerogram: The record from an accelerometer showing acceleration as a function of time.

Acceptable daily intake (ADI): Estimate of the largest amount of chemical to which a person can be exposed on a daily basis that is not anticipated to result in adverse effects (usually expressed in mg/kg/day).

Access control point: A location staffed to restrict entry of unauthorized personnel into a risk area during emergency and/or disaster events. Access control is normally performed just outside of the risk area and involves use of vehicles, barricades, or other measures to deny access to a particular area.

Access shaft: Concrete portion of an outlet works between the shaft house and the gate chamber. The access shaft provides vertical access to the gates.

Accident assessment: The evaluation of the nature, severity, and impact of an accident. Dam operating personnel are primarily responsible for accident assessment for incidents at Reclamation dams.

Acclimation: Adjustment of an organism to a new habitat or environment.

Accretion: Process of growth whereby material is added to the outside of nonliving matter. The gradual increase in flow of a stream attributable to seepage, ground water discharge, or tributary inflow.

Accuracy: How closely an instrument measures the true or actual value of the process variable being measured or sensed.

Acid: A substance that has a pH value between 0 and 7.

Acid rain: Precipitation which has been rendered acidic by airborne pollutants.

Acidic: The condition of water or soil which contains a sufficient amount of acid substances to lower the pH below 7.0.

Acidified: The addition of an acid (usually nitric or sulfuric) to a sample to lower the pH below 2.0.

Acre: Unit for measuring land, equal to 43,560 sq. ft., 4840 sq. yds., or 160 sq. rds.

Acre-foot: A term used in measuring the volume or amount of water needed to cover 1 acre (43,560 square feet) 1 foot deep (325,851 gallons or 1,233.5 cubic meters).

Activated carbon: Adsorptive particles or granules of carbon usually obtained by heating carbon (such as wood). These particles or granules have a high capacity to selectively remove certain trace and soluble materials from water.

Active capacity: The reservoir capacity normally usable for storage and regulation of reservoir inflows to meet established reservoir operating requirements. It extends from the highest of either the top of exclusive flood control capacity, the top of joint use capacity, or the top of active conservation capacity, to the top of inactive capacity. It is also the total capacity less the sum of the inactive and dead capacities. The reservoir capacity that can be used for irrigation, power, municipal and industrial use, fish and wildlife, recreation, water quality, and other purposes.

Active conservation capacity (active storage): The reservoir capacity assigned to regulate reservoir inflow for irrigation, power, municipal and industrial use, fish and wildlife, navigation, recreation, water quality, and other purposes. It does not include exclusive flood control or joint use capacity. It extends from the top of the active conservation capacity to the top of the inactive capacity (or dead capacity where there is no inactive capacity).

Active fault: A fault which, because of its present tectonic setting, can undergo movement from time to time in the immediate geologic future. A fault, which has moved during the recent geologic past (Quarternary) and, thus, may move again. It may or may not generate earthquakes. See capable fault.

Active transport: An energy-expending mechanism by which a cell moves a chemical across the cell membrane from a point of lower concentration to a point of higher concentration, against the diffusion gradient.

Acute: Occurring over a short period of time; used to describe brief exposures and effects which appear promptly after exposure.

Acute exposure: A single exposure to a toxic substance which results in severe biological harm or death. Acute exposures are usually characterized as lasting no longer than a day.

Acute toxicity: The ability of a substance to cause poisonous effects resulting in severe biological harm or death soon after a single exposure or dose. Also, any severe poisonous effect resulting from a single short-term exposure to a toxic substance.

Adaptation: Adjustment to environmental conditions.

Additive effect: Combined effect of two or more chemicals equal to the sum of their individual effects.

Adhesion: Shearing resistance between soil and another material under zero externally applied pressure.

Adit: A nearly horizontal underground excavation in an abutment having an opening in only one end. An opening in the face of a dam for access to galleries or operating chambers.

Adsorbate: The material being removed by the adsorption process.

Adsorbed water: Water in a soil or rock mass, held by physico-chemical forces, having physical properties substantially different from absorbed water or chemically combined water, at the same temperature and pressure.

Adsorbent: The material (for example activated carbon) that is responsible for removing the undesirable substance in the adsorption process.

Adsorption: The process by which chemicals are held on the surface of a mineral or soil particle. The adherence of a gas, liquid, or dissolved material on the surface of a solid. An increase in concentration of gas or solute at the interface of a two-phase system. Should not be confused with absorption.

Advisory Council on Historic Preservation: Executive agency responsible for ensuring requirements of National Historic Preservation Act and 36 CFR Part 800 are met. Visit the Advisory Council on Historic Preservation web site.

Aeolian (eolian): Materials carried, deposited, produced, or eroded by the wind.

Aeolian deposits: Wind- deposited material such as dune sands and loess deposits.

Aerate: To impregnate with gas, usually air.

Aeration: The process of adding air to water by either passing air through water or passing water through air.

Aerial cover: Ground area circumscribed by the perimeter of the branches and leaves of a given plant or group of plants (generally used as a measure of relative density).

Aerobic: A condition in which free (atmospheric) or dissolved oxygen is present in water. The opposite of anaerobic.

Affected environment: Existing biological, physical, social, and economic conditions of an area subject to change, both directly and indirectly, as the result of a proposed human action. Also, the chapter in an environmental impact statement describing current environmental conditions.

Afterbay (tailrace): The body of water immediately downstream from a powerplant or pumping plant. A reservoir or pool that regulates fluctuating discharges from a hydroelectric power plant or a pumping plant.

Afterbay dam (reregulating dam): A dam located downstream from a large hydroelectric powerplant used to regulate discharges downstream. See regulating dam.

Age tank (day tank): A tank used to store a chemical solution of known concentration for feed to a chemical feeder.

Aggradation: Geologic process wherein streambeds, floodplains, sandbars, and the bottom of water bodies are raised in elevation by the deposition of sediment; the opposite of degradation.

Aggregate: Crushed rock or gravel screened to sizes for use in road surfaces, concrete, or bituminous mixes. A mass or cluster of soil particles, often having a characteristic shape.

Agricultural drainage: The process of directing excess water away from root zones by natural or artificial means, such as by using a system of pipes and drains placed below ground surface level (also called subsurface drainage). The water drained away from irrigated farmland.

Agrochemical: Synthetic chemicals (pesticides and fertilizers) used in agricultural production.

Air quality: Measure of the health-related and visual characteristics of the air, often derived from quantitative measurements of the concentrations of specific injurious or contaminating substances.

Air release valve: A valve, usually manually operated, which is used to release air from a pipe or fitting.

Air slaking: The process of breaking up or sloughing when an indurated soil is exposed to air.

Airspace ratio: Ratio of volume of water that can be drained from a saturated soil or rock under the action of force of gravity to total volume of voids.

Air-void ratio: The ratio of the volume of airspace to the total volume of voids in a soil mass.

Air waves: Air borne vibrations caused by explosions.

Alert (Automated Local Evaluation in Real Time): A flood warning system consisting of remote sensors, data transmission by radio, and a computer software package developed by the National Weather Service (NWS). Also, a generic term used for a decision making software package.

Alevin: A young fish which has not yet absorbed its yolk sac.

Algae: Simple plants containing chlorophyll; most live submerged in water. Microscopic plants which contain chlorophyll and live floating or suspended in water. They also may be attached to structures, rocks or other submerged surfaces. They are food for fish and small aquatic animals. Excess algal growths can impart tastes and odors to potable water. Algae produce oxygen during sunlight hours and use oxygen during the night hours. Their biological activities appreciably affect the pH and dissolved oxygen of the water.

Algae bloom: A heavy growth of algae in and on a body of water as a result of high phosphate concentration from farm fertilizers and detergents.

Algal bloom: Rapid and flourishing growth of algae. Sudden, massive growths of microscopic and macroscopic plant life, such as green or bluegreen algae, which develop in lakes and reservoirs.

Algicide: Any substance or chemical specifically formulated to kill or control algae.

Aliquot: Portion of a sample.

Alkali: A soluble salt obtained from the ashes of plants. A substance having marked basic properties. Various soluble salts, principally of sodium, potassium, magnesium, and calcium, that have the property of combining with acids to form neutral salts and may be used in chemical water treatment processes.

Alkali-aggregate reaction (AAR): A deterioration of concrete by which the alkali in the cement paste in the concrete reacts chemically with the silica or carbonate present in some aggregates. In the presence of free moisture, the gel (product of the reaction) will expand and manifest into cracking and differential movement in structures as well as other deleterious effects such as reduction in freeze-thaw durability and compressive and tensile strength. Three forms of alkali-aggregate reaction have been identified, see alkali-silica reaction, the slow/late-expanding type of reaction referred to as alkali-silicate reaction, and the alkali-carbonate reaction. Visit the Alkali-Aggregate Reactions database.

Alkali-carbonate reaction (ACR): Reaction of alkalis which occurs between certain argillaceous dolomitic limestones and the alkaline pore solution in the concrete and causes expansion and extensive cracking. Expansive dolomite limestones are characterized by a matrix of fine calcite and clay minerals with scattered dolomite rhobohedra. This reaction usually occurs early and structures may show cracking within 5 years after construction. See alkali-aggregate reaction.

Alkaline: Having a pH of 7.0 or above. The condition of water or soil which contains a sufficient amount of alkali substances to raise the pH above 7.0. The quality of being bitter due to alkaline content.

Alkali-silica reaction (ASR): Reaction of alkalis with aggregate with various forms of poorly crystalline reactive silica: opal, chert, flint and chalcedony and also tridymite, crystoblite and volcanic glasses. Aggregate containing such materials (e.g., some cherty gravels) may cause deterioration of concrete when present in amounts of 1% to 5%. Concrete made of these aggregates is characterized by the early onset of a relatively rapid expansion. Cracking of structures is often observed within 10 years of construction. See alkali-aggregate reaction.

Alkali-silicate/silica reaction (ASSR): Reaction of alkalis with strained quartz is thought to be one reactive component of aggregates causing this reaction. A wide variety of quartz-bearing rocks have been found to be reactive including graywackes, argillites, quartzwackes, quartzarenites, quartzites, hornfels, quartz biotite, gneiss, granite, phyllite, arkose and sandstone. This type of reaction is characterized by a delayed onset of expansion and cracking may not become evident for up to 20 years after construction. See alkali-aggregate reaction.

Allelopathy: Influence of plants upon each other caused by products of metabolism, e.g., creosote bushes produce a toxic substance which inhibits the growth of other plants in the immediate vicinity.

Allochthonous: Exotic species of a given area. Also refers to deposits of material that originated elsewhere, e.g., drifted plant material on the bottom of a lake.

Allopatric: Having separate and mutually exclusive areas of geographical distribution.

Allowable pile bearing load: The maximum load that can be permitted on a pile with adequate safety against movement of such magnitude that the structure is endangered.

Alluvial: Related to, composed of, or found in alluvium. Sedimentary material transported and deposited by the action of flowing water, such as in a riverbed, flood plain, or delta.

Alluvium: Material transported and deposited by flowing water, such as clay, silt, sand, and gravel. Soil, the constituents of which have been transported in suspension by flowing water and subsequently deposited by sedimentation. A stratified bed of sand, gravel, silt, and clay deposited by flowing water.

Alternating current (AC): An electric current that reverses its direction (positive/negative values) at regular intervals. See direct current.

Alternatives: Courses of action that may meet the objectives of a proposal at varying levels of accomplishment, including the most likely future conditions without the project or action.

Ambient: Surrounding natural conditions or environment at a given place and time. Environmental or surrounding conditions.

Ambient temperature: Temperature of the surrounding air (or other medium).

Ambursen dam: A buttress dam in which the upstream part is a relatively thin flat slab usually made of reinforced concrete.

Ammonium: One form of nitrogen that is usable by plants.

Amperage: The strength of an electric current measured in amperes. The amount of electric current flow, similar to the flow of water in gallons per minute (gpm).

Ampere: A unit of electric current or rate of flow of electrons. One volt across 1 ohm of resistance causes a current flow of 1 ampere.

Amperometric: Based on the electric current that flows between two electrodes in a solution.

Amphibian: Vertebrate animals that have life stages both in water and on land (e.g., salamanders, frogs, and toads). Animals capable of living either in water or land.

Amplification: Modification of the input bedrock ground motion by the overlying unconsolidation materials. Amplification causes the amplitude of the surface ground motion to be increased in some range of frequencies and decreased in others. Amplification is a function of the shear wave velocity and damping of the unconsolidated materials, its thickness and geometry, and the strain level of the input rock motion.

Anadromous: Fish that migrate from salt water to freshwater to breed. Going up rivers to spawn.

Anaerobic: A condition in which free (atmospheric) or dissolved oxygen is not present in water. The opposite of aerobic.

Anchor block. See thrust block.

Angle of external friction (angle of wall friction): Angle between the abscissa and the tangent of the curve representing the relationship of shearing resistance to normal stress acting between soil and surface of another material.

Angle of internal friction (angle of shear resistance): The angle between the axis of normal stress and the tangent to the Mohr envelope at a point representing a given failure-stress condition for solid material.

Angle of obliquity: The angle between the direction of the resultant stress or force acting on a given plane and the normal to that plane.

Angle of repose: Angle between the horizontal and the maximum slope that a soil assumes through natural processes. For dry granular soils, the effect of the height of slope is negligible; for cohesive soils, the effect of height of slope is so great that the angle of repose is meaningless.

Angler-day: The time spent fishing by one person for any part of a day.

Anion: A negatively charged ion in an electrolyte solution, attracted to the anode under the influence of a difference in electrical potential. See cation.

Anisotropic mass: A mass having different properties in different directions at any given point.

Anisotropy: Flow conditions vary with direction. Most aquifers are anisotropic.

Annex (functional): An emergency operations plan element that describes the jurisdiction's plan for functioning in that component area of activity during emergencies.

Annual failure probability: The probability of the load multiplied by the probability of failure.

Annualized loss of life: The sum of the probability of dam failure multiplied by the annual probability of the loading and the estimated number of lives that would be lost (consequences) for each dam failure scenario under a particular loading category (i.e. (probability of failure)(probability of load)(potential loss of life)).

Annular: Ring-shaped.

Annular space: A ring-shaped space located between two circular objects, such as two pipes.

Anode: The positive pole or electrode of an electrolytic system. The anode attracts negatively charged particles or ions (anions). See cathode.

Anoxic: Without oxygen.

Antecedent flood: A flood or series of floods assumed to occur prior to the occurrence of an inflow design flood (IDF).

Anthropogenic: Human-created.

Anticline: A fold in rocks that curves upward in a convex way.

Appendix: An emergency operations plan element attached to a functional annex to provide information on special approaches or requirements generated by unique characteristics of specified hazards of particular concern to the jurisdiction.

Application efficiency: The ratio of the average depth of irrigation water infiltrated and stored in the root zone to the average depth of irrigation water applied, expressed as a percent.

Applied water (delivered water): Water delivered to a user. Applied water may be used for either inside uses or outside watering. It does not include precipitation or distribution losses. It may apply to metered or unmetered deliveries.

Appraisal estimate: An estimate used in an appraisal study as an aid in selecting the most economical plan by comparing alternative features or for determinimg whether more detailed investigations of a potential project are economically justified. Used to obtain approximate costs in a short period of time with inadequate data. Not to be used for project authorization.

Appraisal level of detail: The level of detail necessary to facilitate making decisions on whether or not to proceed with a detailed study and evaluation of any alternative.

Appraisal study (appraisal report): A study incorporating an appraisal level of detail.

Approach channel: The channel upstream from that portion of the spillway having a concrete lining or concrete structure. Channel upstream from intake structure of an outlet works. Channel is generally unlined, excavated in rock or soil, with or without riprap, soil cement or other types of erosion protection.

Appropriation: Amount of water legally set apart or assigned to a particular purpose or use.

Appropriative: Water rights to or ownership of a water supply which is acquired for the beneficial use of water by following a specific legal procedure.

Appurtentant structures: Outlet works, spillways, bridges, drain systems, tunnels, towers, etc.

Apron (fore apron): A section of concrete or riprap constructed upstream or downstream from a control structure to prevent undercutting of the structure. A short ramp with a slight pitch. A floor or lining of concrete, timber, or other suitable material at the toe of a dam, discharge side of a spillway, a chute, or other discharge structure, to protect the waterway from erosion from falling water or turbulent flow.

Aquatic: Living, growing, or occurring in or on the water.

Aquatic algae: Microscopic plants that grow in sunlit water containing phosphates, nitrates, and other nutrients.

Aqueduct: Man-made canal or pipeline used to transport water.

Aqueous: Something made up of, similar to, or containing water; watery.

Aquiclude: A layer of clay which limits the movement of ground water.

Aquifer: A water-bearing stratum of permeable rock, sand, or gravel. A water-bearing formation that provides a ground water reservoir. Underground water-bearing geologic formation or structure. A geologic formation, group of formations, or part of a formation that stores and transmits water and yields significant quantities of water to wells and springs. A natural underground layer of porous, water-bearing materials (sand, gravel) usually capable of yielding a large amount or supply of water.

Arable: Suitable for farming. Having soil or topographic features suitable for cultivation.

Arable land: Land which when farmed in adequate size units for the prevailing climatic and economic setting, and provided with the essential on-farm improvements of removing vegetation, leveling, soil reclamation, drainage, and irrigation related facilities, will generate sufficient income under irrigation to pay all farm production expenses; provide a reasonable return to the farm family's labor, management, and capital; and at least pay the operation, maintenance, and replacement costs of associated irrigation and drainage facilities.

Arch dam: A concrete or masonry dam which is curved upstream in plan so as to transmit the major part of the water load to the abutments and to keep the dam in compression. A solid concrete dam curved upstream in plan. An arch dam is most likely used in a narrow site with steep walls of sound rock. See thin arch dam, medium-thick arch dam, thick arch dam, arch-buttress dam, arch-gravity dam, constant angle arch dam, constant radius arch dam, double curvature arch dam, and multiple arch dam.

Arch-buttress dam (or curved buttress dam): A buttress dam which is curved in plan.

Arch-gravity dam: An arch dam which is only slightly thinner than a gravity dam.

Archaic: In American archeology, a cultural stage following the earliest known human occupation in the New World (about 5,500 B.C. to A.D. 100). This stage was characterized by a generalized hunting and gathering lifestyle and seasonal movement to take advantage of a variety of resources.

Archeology: Study of human cultures through the recovery and analysis of their material relics.

Archimedean screw: An ancient water-raising device attributed to Archimedes, made up of a spiral tube coiled about a shaft or of a large screw in a cylinder, revolved by hand. A pump consisting of an inclined, revolving, corkscrew-shaped shaft tightly enclosed in a pipe.

Arching: The transfer of stress from a yielding part of a soil or rock mass to adjoining less-yielding or restrained parts of the mass.

Area-capacity table: A table giving reservoir storage capacity, and sometimes surface areas, in terms of elevation increments.

Area of influence of a well: Area surrounding a well within which the piezometric surface has been lowered when pumping has produced a maximum steady rate of flow.

Arid: A term describing a climate or region in which precipitation is so deficient in quantity or occurs so infrequently that intensive agricultural production is not possible without irrigation.

Armoring. See riprap.

Arroyo: A gully or channel cut by an intermittent stream. A water-carved channel or gulley in an arid area, usually rather small in cross section with steep banks, dry much of the time due to infrequent rainfall and the depth of the cut which does not penetrate below the level of permanent ground water.

Artesian well: Water held under pressure in porous rock or soil confined by impermeable geologic formations. An artesian well is free flowing. See confined aquifer.

Artifact: A human-made object.

Artificial drains: Man-made or constructed drains.

Artificial recharge: Addition of surface water to a ground water reservoir by human activity, such as putting surface water into spreading basins. See ground water recharge, and recharge basin.

Associated facility: A term used by Reclamation to describe those facilities examined by the respective regional or area office. These facilities include most carriage, distribution, and drainage systems, small diversion works, small pumping plants and powerplants, open and closed conduits, tunnels, siphons, small regulating reservoirs, waterways, and class B bridges.

Association of Boards of Certification (ABC): An international organization representing over 150 boards which certify the operators of waterworks and waste water facilities. For more information, visit the ABC website.

Asymmetric: Not similar in size, shape, form or arrangement of parts on opposite sides of a line, point or plane.

Atmospheric pressure: Pressure of air enveloping the earth, averaged as 14.7 psi at sea level, or 29.92 inches of mercury as measured by a standard barometer.

Attenuation: Decrease in amplitude of the seismic waves with distance due to geometric spreading, energy absorption and scattering.

Attribute survey: Survey to determine the important components of the recreational experience.

Atypical: Not typical.

Auger: A rotating drill having a screw thread that carries cuttings away from the face.

Author's signature: This is the signature of the person or persons with primary responsibility for writing the document. Signature of the document by the author(s) signifies that a draft document was provided to team members and that they had an opportunity to comment on the draft. The author's signature also implies that comments were considered and that any critical issues or influencing factors were incorporated into the document.

Authorization: An act by the Congress of the United States which authorizes use of public funds to carry out a prescribed action.

Automatic generation control (AGC): Computerized power system regulation to maintain scheduled generation within a prescribed area in response to changes in transmission system operational characteristics.

Auxiliary spillway: A spillway, usually located in a saddle or depression in the reservoir rim which leads to a natural or excavated waterway, located away from the dam which permits the planned release of excess flood flow beyond the capacity of the service spillway. A control structure is seldom furnished. The crest is set at the maximum water surface elevation for a 100-year flood or some other specific frequency flood. The auxiliary spillway thus has only infrequent use. Any secondary spillway which is designed to be operated very infrequently and possibly in anticipation of some degree of structural damage or erosion to the spillway during operation.

Available capacity: The amount of water held in the soil that is available to the plants. See water holding capacity.

Average: The arithmetic mean. The sum of the values divided by the number of values.

Average annual runoff: For a specified area, the average value of annual runoff amounts calculated for a selected period of record that represents average hydrologic conditions.

Average year water demand: Demand for water under average hydrologic conditions for a defined level of development.

Average year supply: The average annual supply of a water development system over a long period.

Avian: Having to do with birds.

Avoirdupois weight: An English and American system of weights based on a pound of 16 ounces.

Axis: A straight line around which a shaft or body revolves.

Axis of dam: A vertical plane or curved surface, appearing as a line in plan or cross section, to which horizontal dimensions can be referred.

Axis of dam (concrete): A vertical reference surface coincident with the upstream face at the top of the dam.

Back pressure: A pressure that can cause water to backflow into the water supply when a user's water system is at a higher pressure than the public water system.

Backfill: Material used in refilling excavation, or the process of such refilling. Material used to fill an excavated trench.

Backfill concrete: Concrete used in refilling excavation in lieu of earth material.

Backflow: A reverse flow condition, created by a difference in water pressures, which causes water to flow back into the distribution system.

Backfurrow: The first cut of a plow, from which the slice is laid on undisturbed soil.

Backsiphonage: A form of backflow caused by a negative or below atmospheric pressure within a water system.

Backwater: A small, generally shallow body of water with little or no current of its own. Stagnant water in a small stream or inlet. Water moved backward or held back by a dam, tide, etc.

Baffle: A flat board or plate, deflector, guide or similar device constructed or placed in flowing water to cause more uniform flow velocities, to absorb energy, and to divert, guide, or agitate the flow.

Baffle block (dentate): One of a series of upright obstructions designed to dissipate energy as in the case of a stilling basin or drop structure. A block, usually of concrete, constructed in a channel or stilling basin to dissipate the energy of water flowing at high velocity.

Bailer: A 10- to 20-foot-long pipe equipped with a valve at the lower end. A bailer is used to remove slurry from the bottom or the side of a well as it is being drilled.

Balanced head condition: The condition in which the water pressure on the upstream and downstream sides of an object are equal (such as an emergency or regulating gate).

Ball-milling: The repeated churning action of cobbles, gravel, and sand caused by the force of water in a stilling basin or other structure by which severe concrete abrasion can occur.

Bank full: An established river stage at a given location along a river which is intended to represent the maximum safe water level that will not overflow the river banks or cause any significant damage within the river reach.

Bank storage: Water that has inflitrated from a reservoir into the surrounding land where it remains in storage until water level in the reservoir is lowered.

Barrage (gate-structure dam): A barrier built across a river, comprising a series of gates which when fully open allow the flood to pass without appreciably increasing the flood level upstream of the barrage.

Bascule gate. See flap gate.

Base: A substance that has a pH value between 7 and 14.

Base course: A layer of specified or selected material of planned thickness constructed on the subgrade or subbase for the purpose of serving one or more functions such as distributing load, providing drainage, minimizing frost action, etc.

Base flood: The flood having a one percent chance of being equalled or exceeded in any given year. This term is used in the National Flood Insurance Program (NFIP) to indicate the minimum level of flooding to be used by a community in its flood plain management regulations.

Base flood plain: The flood plain innundated by the 100-year flood.

Base flow: Ground water inflow to the river. Portion of stream discharge that is derived from natural storage.

Base safety condition (BSC): The level of loading above which a dam failure does not contribute an incremental loss of life.

Base width or thickness: The maximum thickness or width of a dam measured horizontally between upstream and downstream faces and normal to the axis or centerline crest of the dam, but excluding projections for outlets or other appurtenant structures. The base thickness of the crown cantilever of an arch dam. In general, the term thickness is used for gravity or arch dams, and width is used for other dams.

Baseline (condition or alternative): Conditions that would prevail if no actions were taken. See future without.

Baseline profile: Used for a survey of the environmental conditions and organisms existing in a region prior to unnatural disturbances.

Baseload: Minimum load in a power system over a given period of time.

Baseloading: Running water through a powerplant at a roughly steady rate, thereby producing power at a steady rate.

Baseload plant: Powerplant normally operated to carry baseload; consequently, it operates essentially at a constant load.

Basin runoff model: Any one of the computer programs that mathematically models basin characteristics to forecast reservoir inflow from rainfall and/or streamflow data.

Batter: Inclination from the vertical. A pile driven at an angle to widen the area of support and to resist thrust.

Beaching: The action of water waves by which beach materials settle into the water because of removal of finer materials.

Bear-trap gate: Any of a family of crest gates consisting of two leaves, an upstream leaf hinged and sealed at the upstream edge and a downstream leaf hinged and sealed on its downstream edge. The two leaves have a sliding seal at their juncture. When lowered the leaves are in a horizontal position. Gate is raised by admitting water from the forebay into the space beneath the leaves. Probably the first gate operated on the principal of application of headwater pressure.

Bed elevation: Height of streambed above a specified level.

Bed layer: The flow layer, several grain diameters thick (usually taken as two grain diameters thick), immediately above the bed.

Bed load: Sediment that moves by rolling or sliding along the bed and is essentially in contact with the streambed in the bed layer.

Bed-load discharge: The quantity of bed load passing a cross section of a stream in a unit of time.

Bed material: Unconsolidated material, or sediment mixture, of which a streambed is composed.

Bed-material discharge: That part of the total sediment discharge which is composed of grain sizes found in the bed. The bed-material discharge is assumed equal to the transport capability of the flow.

Bedding: Ground, or layer of such, for support purposes on which pipe is laid. Soil placed beneath and beside a pipe to support the load on the pipe.

Bedding plane: A separation or weakness between two layers of rock, caused by changes during the building up of the rock-forming material.

Bedload: Coarse sediments carried along near the bottom of a river.

Bedrock: The solid rock at the surface or underlying other surface materials. Rock of relatively great thickness and extent in its native location. A general term for any solid rock, not exhibiting soil-like properties, that underlies soil or other unconsolidated surficial materials. As distinguished from boulders. The consolidated body of natural solid mineral matter which underlies the overburden soils. The solid rock that underlies all soil, sand, clay, gravel, and other loose materials on the earth's surface. Any sedimentary, igneous, or metamorphic material represented as a unit in geology; being a sound and solid mass, layer, or ledge of mineral matter; and with shear wave velocities greater than 2500 feet per second.

Behavior: Reaction of an animal to its environment.

Bell: An expanded, or enlarged, end of a pipe section, into which the next pipe fits. See spigot.

Bench: A working level or step in a cut.

Bench mark (BM): A permanent or temporary monument of known elevation above sea level, used for vertical control at a construction site. A point of known or assumed elevation used as a reference in determining other elevations. A permanent reference point (elevation) used in a survey.

Bend: A change of direction in piping.

Benefit-cost ratio: The ratio of the present value of project benefits to the present value of the project costs, used in economic analysis.

Beneficial use: Water loss through use for the betterment of society, e.g. irrigation or municipal use. See consumptive use.

Beneficiary: Any individual, entity, or governmental agency (local, State, or Federal) that benefits from a Reclamation project.

Benthic: Bottom of rivers, lakes, or oceans; organisms that live on the bottom of water bodies. Bottom- or depth-inhabiting.

Benthos: Organisms living in or on the bottom of a lake, pond, ocean, stream, etc.

Bentonitic clay: A clay with a high content of the mineral montmorillonite, usually characterized by high swelling on wetting.

Berm: A horizontal strip or shelf built into an embankment or cut to break the continuity of the slope, usually for the purpose of reducing erosion or to increase the thickness of the embankment at a point of change in a slope or defined water surface elevation. A horizontal step in the sloping profile of an embankment dam. A shelf that breaks the continuity of a slope, or artificial ridge of earth. A ledge or shoulder, as along the edge of a road or canal. An artificial ridge of earth.

Biennial: Plant which produces seeds during its second year and then dies.

Binder (soil binder): Portion of soil passing a No. 40 United States standard sieve.

Binomial:: Scientific name of plants or animals which has two parts: a genus and a species name.

Bioaccumulation: The intake and retention of nonfood substances by a living organism from its environment, resulting in a build-up of the substances in the organism.

Bioassay: Test which determines the effect of a chemical on a living organism.

Bioassimilation: The accumulation of a substance within a habitat.

Biological diversity: Number and kinds of organisms per unit area or volume; the composition of species in a given area at a given time.

Biological growth: The activity and growth of any and all living organisms.

Biological magnification (biomagnification): Step by step concentration of substances in successive levels of food chains. The enhancement of a substance (usually a contaminant) in a food web such that the organisms eventually contain higher concentrations of the substance than their food sources.

Biological opinion: Document stating the U.S. Fish and Wildlife Service (FWS) and the National Marine Fisheries Service (NMFS) opinion as to whether a Federal action is likely to jeopardize the continued existence of a threatened or endangered species or result in the destruction or adverse modification of critical habitat.

Biological processes: Processes characteristic of, or resulting from, the activities of living organisms.

Biology: The scientific study of life.

Biomass: Total mass or amount of living organisms in a particular area or environment.

Biosphere: Portion of the solid and liquid earth where organisms live.

Biota: Plant and animal life of a region.

Biotic potential: Inherent capacity of an organism to reproduce and survive, which is pitted against limiting influences of the environment.

Biotic pyramid: Set of all food chains or hierarchic arrangements of organisms as eaters and eaten in a prescribed area when tabulated by numbers or by biomasses, usually takes the form of an inverted pyramid.

Biotope: Smallest geographical unit of a habitat, characterized by a high degree of uniformity in the environment and its plant and animal life, e.g., a decaying stump.

Biotype: Genetically homogeneous population composed only of closely similar individuals; a genotypic race or group of organisms.

Bituminous: Containing asphalt or tar.

Blade: Usually a part of an excavator which digs and pushes dirt but does not carry it.

Blast: To loosen or move rock or soil by means of explosives or an explosion.

Blasting mats: A blanket usually composed of woven cable or interlocked rings placed over a blast to reduce flyrock.

Blockloading: Providing a consistent amount of electrical power in a stated period of time.

Blue tops: Grade stakes whose tops indicate finish grade level.

Bonnet: The upper portion or the cover of a gate valve body.

Borderline soils: Soils that have the characteristics of two of the classification groups in the Unified Soil Classification System.

Boring: Rotary drilling.

Borrow: Material excavated from one area to be used as fill material in another area.

Borrow areas: Generally, surface areas, that contain borrow pits. The area from which material for an embankment is excavated.

Borrow pits: Specific site(s) within a borrow area from which material is excavated for use.

Boulder: A rock fragment, usually rounded by weathering or abrasion, with an average dimension of 12 inches or more: will not pass a 12-inch screen. A rock which is too heavy to be lifted readily by hand.

Boulder clay: A geological term used to designate glacial drift that has not been subjected to the sorting action of water and therefore contains particles from boulders to clay sizes.

Box girder: A hollow steel beam with a square or rectangular cross section.

Brackish: Mixed fresh and salt waters. Water containing too much salt to be useful to people but less salt than ocean water.

Brake horsepower: The brake horsepower of a pump is the actual motor input horsepower required to produce the hydraulic horsepower from a pump (flow and head) taking into account the losses incurred within the pump due to friction, leakage, etc. Brake horsepower is the ratio of hydraulic horsepower to pump efficiency.

Branch: An addition to the main pipe in a piping system.

Breach: A gap, rift, hole, or rupture in a dam; providing a break; allowing water stored behind a dam to flow through in an uncontrolled and unplanned manner. An eroded opening through a dam which drains the reservoir. A controlled breach is a constructed opening. An uncontrolled breach is an unintentional opening which allows uncontrolled discharge from the reservoir.

Breach hydrograph: A flood hydrograph resulting from a dam breach.

Brecciated: A rock made up of highly angular, coarse fragments.

Breeding density: Density of sexually mature organisms in a given area during the breeding period.

Breeding potential: Maximum rate of increase in numbers of individuals of a species or population under optimum conditions.

Breeding rate: Actual rate of increase of new individuals in a given population; the breeding potential minus limiting factors.

Bridge, Class A: A bridge located on a Federal-aid highway. Examination of this type of bridge is required every 2 years with an in-depth examination every 6 years.

Bridge, Class B: A bridge not located on a Federal-aid highway that is used by the public. Examination of this type of bridge is required every 3 years.

Bridge, Class C: A bridge not located on a Federal-aid highway that can be designated as being a private or operating bridge and could be gated or closed to the public, if desired.

British thermal units (Btu): Quantity of heat required to raise the temperature of one avoirdupois pound of water 1 degree F at or near 39.2 degrees F.

Broad-crested weir: An overflow structure on which the nappe is supported for an appreciable length in the direction of flow.

Bucket: A part of an excavator which digs, lifts, and carries dirt.

Buffer strips (filter strips, vegetated filter strips, grassed buffers): Strips of grass or other close-growing vegetation that separates a waterway (ditch, stream, creek) from an intensive land use area (subdivision, farm).

Built environment: Human- modified environment, e.g. buildings, roads, and cities. See cultural resource.

Bulkhead: A one-piece fabricated steel unit which is lowered into guides and seals against a frame to close a water passage in a dam, conduit, spillway, etc. An object used to isolate a portion of a waterway for examination, maintenance, or repair. A wall or partition erected to resist ground or water pressure.

Bulkhead gate: A gate used either for temporary closure of a channel or conduit before dewatering it for inspection or maintenance or for closure against flowing water when the head difference is small (e.g., for diversion tunnel closure).

Bulking: The increase in volume of a material due to manipulation. Rock bulks upon being excavated; damp sand bulks if loosely deposited, as by dumping, because the "apparent cohesion" prevents movement of the soil particles to form a reduced volume.

Bureau of Indian Affairs (BIA): The Bureau of Indian Affairs' mission is to enhance the quality of life, to promote economic opportunity, and to carry out the responsibility to protect and improve the trust assets of American Indians, Indian tribes, and Alaska Natives. BIA is an agency within the U.S. Department of the Interior.

Bureau of Land Management (BLM): The Bureau of Land Management, an agency within the U.S. Department of the Interior, administers 264 million acres of America's public lands, located primarily in 12 Western States. The BLM sustains the health, diversity, and productivity of the public lands for the use and enjoyment of present and future generations.

Burden: In blasting, the distance between the free face and the first row of holes or the distance between rows of holes parallel to the face. Apparent burden is the burden as outlined by the delay pattern.

Butt joint (open joint): In pipe, flat ends that meet but do not overlap.

Butterfly valve: A valve designed for quick closure that consists of a circular leaf, slightly convex in form, mounted on a transverse shaft carried by two bearings.

Buttress dam: A dam consisting of a watertight upstream part (such as a concrete sloping slab) supported at intervals on the downstream side by a series of buttresses (walls normal to the axis of the dam). Buttress dams can take many forms. See arch-buttress dam, flat slab or slab and buttress dam, massive head buttress dam, multiple arch dam, and solid head buttress dam.

Cairn: A pile of stones used as a marker.

Caisson: A box or chamber used in construction work under water. A structure or chamber which is usually sunk or lowered by digging from the inside. Used to gain access to the bottom of a stream or other body of water.

Camber: The extra height added to the crest of embankment dams to ensure that the freeboard will not be diminished by foundation settlement or embankment consolidation. The amount of camber is different for each dam and is dependent on the amount of foundation settlement and embankment expected to occur.

Canal: A channel, usually open, that conveys water by gravity to farms, municipalities, etc.

Canal headworks: The beginning of a canal.

Canal prism: The shape of the canal as seen in cross section.

Candidate species: Plant or animal species that are candidates for designation as endangered (in danger of becoming extinct) or threatened (likely to become endangered), but is undergoing status review by the U.S. Fish and Wildlife Service (FWS).

Capable fault: An active fault that is judged capable of producing macro-earthquakes and exhibits one or more of the following characteristics:

(1) Movement at or near the ground surface at least once within the past 35,000 years.

(2) Macro-seismicity (3.5 magnitude Richter or greater) instrumentally determined with records of sufficient precision to demonstrate a direct relationship with the fault.

(3) A structural relationship to a capable fault such that movement on one fault could be reasonably expected to cause movement on the other.

(4) Established patterns of micro-seismicity that define a fault, with historic macro-seismicity that can reasonably be associated with that fault.

Capacity: In power terminology, the load for which a generator, transmission line, or system is rated, expressed in kilowatts. The maximum load that a machine, station, or system can carry under existing service conditions. Equivalent terms: peak capability, peak generation, firm peak load, carrying capability. In transmission, the maximum load a transmission line is capable of carrying. See excess capacity and peaking capacity. Also refers to powerplant generation capability under specific operating conditions and the amount of marketable resource under such conditions.

Capillary action (capillarity): The rise or movement of water in the interstices of a soil or rock due to capillary forces. The process by which water rises through rock, sediment or soil caused by the cohesion between water molecules and an adhesion between water and other materials that pulls the water upward. A property of surface tension that draws water upwards. See capillary movement.

Capillary attraction (capillary force): The tendency of water to move into fine spaces, as between soil particles, regardless of gravity.

Capillary forces: The molecular forces which cause the movement of water through very small spaces.

Capillary fringe zone: The zone above the free water elevation in which water is held by capillary action. The porous material just above the water table which may hold water by capillarity in the smaller void spaces.

Capillary head: The potential, expressed in head of water, that causes the water to flow by capillary action.

Capillary migration (capillary flow): The movement of water by capillary action.

Capillary movement: Movement of underground water in response to capillary attraction. See capillary action.

Capillary rise: The height above a free water elevation to which water will rise by capillary action.

Capillary water: Underground water held above the water table by capillary attraction. Water subject to the influence of capillary action.

Capital costs: Costs (usually long-term debt) of financing construction and equipment. Capital costs are usually fixed, one-time expenses which are independent of the amount of water produced. All the implements, equipment, machinery and inventory used in the production of goods and services.

Carnivore: Any flesh-eating or predatory organism.

Carry over: The quantity of water which continues past an inlet.

Cartography: Art and science of graphically representing the features of the Earth's surface; synonymous with map making.

Casing: A pipe lining for a drilled hole. The material that is installed in wells to prevent the collapse of the walls of the bore hole, to prevent pollutants from entering the well, and to house the pump and pipes.

Catastrophe: A sudden and great disaster causing misfortune, destruction, or irreplaceable loss extensive enough to cripple activities in an area.

Catch: At a recreational fishery, refers to the number of fish captured, whether they are kept or released.

Catchment basin: Unit watershed; an area from which all the drainage water passes into one stream or other body of water.

Cathode: The negative pole or electrode of an electrolytic cell or system. The cathode attracts positively charged particles or ions (cations). See anode.

Cathodic protection: An electrical system for prevention of rust, corrosion, and pitting of metal surfaces which are in contact with water or soil. A low-voltage current is made to flow through a liquid (water) or a soil in contact with the metal in such a manner that the external electromotive force renders the metal structure cathodic. This concentrates corrosion on auxiliary anodic parts which are deliberately allowed to corrode instead of letting the structure corrode.

Cation: A positively charged ion in an electrolyte solution, attracted to the cathode under the influence of a difference in electrical potential. See anion.

Caulk: The material used to seal joints.

Cavitation: The formation of partial vacuums in fast-flowing water caused by subatmospheric pressures immediately downstream from an obstruction or offset. Usually accompanied by noise and vibration. The formation of voids or cavities caused in a liquid due to turbulence or temperature which causes the pressure in local zones of the liquid to fall below the vapor pressure. This happens on the backside of ship propellers, water turbines, blades in pumps, in high-velocity flow lines, and similar locations, depending on the design of equipment and degree of turbulence. The formation and collapse of a gas pocket or bubble on the blade of an impeller or the gate of a valve. The collapse of this gas pocket or bubble drives water into the impeller or gate with a terrific force that can cause pitting on the impeller or gate surface. Cavitation is accompanied by loud noises that sound like someone is pounding on the impeller or gate with a hammer. The attack on surfaces caused by subatmospheric pressures immediately downstream from an obstruction or offset. Usually accompanied by noise and vibration.

Cavitation damage: Damage caused when partial vacuums formed in a liquid by a swiftly moving solid body (e.g. a propeller) pit and wear away solid surfaces (e.g. metal or concrete). The attack on surfaces caused by the implosion of bubbles of water vapor.

Cellular gravity dam. See hollow gravity dam.

Celsius (C): Unit of temperature. Degrees Celsius equals (5/9)x(degrees Fahrenheit-32).

Central flyway: An important international migration route for many birds.

Centrifugal pump: A pump that moves water by centrifugal force developed by rapid rotation of an impeller. A pump consisting of an impeller fixed on a rotating shaft that is enclosed in a casing, and having an inlet and discharge connection. As the rotating impeller whirls the water around, centrifugal force builds up enough pressure to force the water through the discharge outlet. The rotating impeller imparts energy to the water.

Certification signature: Certification signatures are those of the persons who co-facilitated the risk analysis. These signatures signify that Reclamation methodology, processes, and requirements were followed. In addition, these signatures verify that qualifications of the persons making various probability estimates were appropriate. The purpose of endorsing qualifications is to reduce the potential for inappropriate estimates, or conflicts, arising from limited qualifications that might result in total rejection of risk analysis findings. Certification signatures also signify that the spirit of the risk analysis and team dynamics are represented by the document. In other words, any divergent views, critical issues or significant influencing factors have been captured. This is a check of the author's responsibility to fully capture and represent the team's thinking.

Certified water right: A State-issued document that serves as legal evidence that an approved application has been physically developed and the water put to beneficial use. The certificate establishes: priority date, type of beneficial use, and the maximum amount of water that can be used. Verification must be provided to the State through a survey conducted by an approved water-rights examiner. Even certified rights are subject to occasional review to ensure continued beneficial use.

Chamfer: To bevel or slope an edge or corner.

Channel: Natural or artificial watercourse of perceptible extent, with a definite bed and banks to confine and conduct continuously or periodically flowing water. Rivers and streams. A general term for any natural or artificial facility for conveying water.

Channel margin deposits: Narrow sand deposits which line channel banks.

Check dam: A small dam designed to retard the flow of water and sediment in a channel, used especially to control soil erosion. Small barrier constructed in a gully or other small watercourse to decrease flow velocity, minimize channel scour, and promote deposition of sediment.

Check structure: A structure used to regulate the upstream water surface and control the downstream flow in a canal.

Check valve: Any device which will allow fluid or air to pass through it in only one direction. A special valve with a hinged disc or flap that opens in the direction of normal flow and is forced shut when flows attempt to go in the reverse or opposite direction of normal flow. A device preventing backflow in pipes. Water can flow readily in one direction but any reversal of the flow causes the check valve to close.

Checked signature: Checked signatures verify that all probability estimates, inputs and outputs and their distributions, were entered correctly into event trees, and that any other calculations, figures, or tables have been checked. This includes "back-of-the-envelope" calculations performed during the risk analysis but not documented in any place but the report. In addition, the accuracy of computer spreadsheets have been checked.

Chimney drain: A vertical or inclined layer of pervious material in an embankment to facilitate and control drainage of the embankment fill.

Chironomid: Group of two-winged flying insects who live their larval stage underwater and emerge to fly about as adults.

Chipping: Loosening of shallow rock by light blasting or air hammers.

Chisel plowing: Cropland preparation by a special implement (chisel) that avoids complete inversion of the soil (as occurs with conventional moldboard plowing). Chisel plowing can leave a protective cover of crop residues on the soil surface that helps prevent erosion and improve infiltration.

Chlorination: The application of chlorine to water, generally for the purpose of disinfection, but frequently for accomplishing other biological or chemical results (aiding coagulation and controlling tastes and odors).

Chute: Portion of spillway between the gate or crest structure and the terminal structure, where open- channel flow conditions will exist. A conduit for conveying free-flowing materials at high velocity to lower elevations.

Cipolletti weir (trapezoidal weir): A contracted weir of trapezoidal shape in which the sides of the notch are given a slope of 1 horizontal to 4 vertical.

Circle of influence: The circular outer edge of a depression produced in the water table by the pumping of water from a well. See cone of influence.

Circuit: The complete path of an electric current, including the generating apparatus or other source; or, a specific segment or section of the complete path.

Circuit breaker: A safety device in an electrical circuit that automatically shuts off the circuit when it becomes overloaded. The device can be manually reset.

Circuit mile: For single circuit electric power transmission lines, circuit miles are equal to geographic miles or pole miles. For double circuit transmission lines, the number of circuit miles is twice the structure, pole, or geographic miles.

Cistern: A small tank (usually covered) or a storage facility used to store water for a home or farm. Often used to store rain water.

Civil Defense Agency: State and/or local agency responsible for emergency operations, planning, mitigation, preparedness, response, and recovery for all hazards. Usually, the more current term of emergency management agency is used.

Cladophora: Filamentous green alga.

Clam shell gate: High pressure regulating gate consisting of two curved leaves which open and close over the end of a conduit. Used for free discharge into air with minimal cavitation damage.

Class (pipe and fittings): The working pressure rating of a specific pipe for use in water distribution systems which includes allowances for surges. This term is used for cast iron, ductile iron, asbestos cement, and some plastic pipe.

Clay: Fine-grained soil or the fine-grained portion of soil that can be made to exhibit plasticity (putty-like properties) within a range of water contents, and that exhibits considerable strength when air-dry. Plastic soil which passes a No. 200 United States Standard sieve.

Clearance: A procedure used to establish, under tightly controlled discipline, a safe environment for maintenance, repair, or inspection. It includes systematically isolating pertinent equipment from all sources of hazardous energy (hydraulic, electrical, mechanical, pneumatic, chemical, etc.) and attaching safety tags or locks to the appropriate controls. Also, it includes a written statement that documents isolation of the equipment (also referred to as lockout or tagout).

Clearing: The removal of all vegetation such as trees, shrubs, brush, stumps, exposed roots, down timber, branches, grass, and weeds. The removal of all rubbish and all other objectionable material. See grubbing.

Climate: Average conditions of the weather over a number of years. See macroclimate and microclimate.

Climatic year: Continuous 12- month period during which a complete climactic cycle occurs.

Coarse gravel protection: Gravel generally placed in a layer upon a finished surface to protect the finished surface from deterioration or erosion.

Coaster gate: A rectangular gate similar in construction and appearance to the tractor gate except sealing is accomplished in much the same way as a ring seal gate.

Coating: The protective material applied to the outer surface of a material, frequently metalwork.

Cobble (cobblestone): A rock fragment, usually rounded or semirounded, with an average dimension between 3 to 12 inches; will pass a 12-inch screen, but not a 3-inch screen.

Cofferdam: A temporary structure enclosing all or part of the construction area so that construction can proceed in the dry. A diversion cofferdam diverts a river into a pipe, channel or tunnel. A temporary barrier, usually an earthen dike, constructed around a worksite in a reservoir or on a stream, so the worksite can be dewatered or the water level controlled. See dam.

Cohesion: The mutual attraction of soil particles due to molecular and capillary forces in the presence of water. Cohesion is high in clay (especially dry) but of little significance in silt or sand. The ability of a substance to stick to itself and pull itself together. Molecular attraction which holds two particles together.

Cohesionless materials (cohesionless soil): Soil materials that when unconfined have little or no strength when air-dried and that have little or no cohesion when submerged. Soil that has little tendency to stick together whether wet or dry, such as sands and gravels.

Cohesive soil: Predominantly clay and silt soil, fine- grained particles, that sticks together whether wet or dry. A soil that when unconfined has considerable strength when air-dried, and that has significant cohesion when submerged.

Cold joint: An unplanned joint resulting when a concrete surface hardens before the next batch is placed against it. Fresh concrete placed on harden concrete.

Cold-water fishery: Water or water system which has an environment suitable for salmonoid fishes.

Coliform: Organisms common to the intestinal tract of humans and animals; the organisms' presence in waste water is an indicator of pollution.

Collar: The open end of a drill hole.

Collaring: Starting a drill hole. When the hole is deep enough to hold the bit from slipping out of it, it is said to be collared.

Collodial suspension: A method of sediment transport in which water turbulence (movement) supports the weight of the sediment particles, thereby keeping them from settling out or being deposited.

Colluvium: A general term applied to loose and incoherent deposits, usually at the foot of a slope and brought there chiefly by gravity.

Commercial river trip: Trips organized by boating companies that conduct tours for paying passengers or customers.

Commercial user day: Amount of commercial use within a 24-hour period or any portion thereof.

Commercial water use: Water for motels, hotels, restaurants, office buildings, other commercial facilities, and institutions.

Common excavation: All materials excavated not considered as rock. Boulders or detached pieces of solid rock less than 1 cubic yard in volume are classified as common excavation. See excavation.

Common material: All earth materials which do not fall under the definition of rock.

Community: All members of a specified group of species present in a specific area at a specific time; a group of people that see themselves as a unit.

Community cohesion: Ability of a community to have a unified response when facing a problem; e.g., an external threat to their sustainability.

Compacted backfill: Backfill which has been reduced to bulk by rolling, tamping, or soaking.

Compacted embankment: Embankment which has been reduced in bulk by rolling, tapping, or soaking.

Compaction: To make soil dense by mechanical manipulation. Mechanical action which increases the density by reducing the voids in a material. See dumped, dynamic compaction, jetting, ponding, puddling, rolling, saturation, sluicing, surface vibration, and tamping.

Compaction curve (Proctor curve, moisture-density curve): The curve showing the relationship between the dry density and the water content of a soil for a given compactive effort.

Comprehensive Facility Review (CFS): A review performed on a high- or significant-hazard dam every 6 years which includes a field examination and a state-of-the-art review of a structure's design assumptions, construction practices, and integrity under various loading conditions. A detailed examination performed on dams with a senior dam engineer. Comprehensive facility reviews are generally followed every 3 years by a periodic facility review.

Compressibility: Property of a soil describing its susceptibility to decrease in volume when subjected to load.

Computer base station: Computer equipment that is designated to receive data from the data collection platforms and translate that data into useable information for the decision makers.

Concentrated flow path: An existing hypothetical avenue of concentrated seepage. The descriptor "concentrated" implies sufficient energy to carry/move material and erode. Energy varies with material type, construction methods, geology, etc., since an anomaly may result in higher velocities adjacent to materials than more uniform flow through a given material.

Concrete dam: See arch dam, buttress dam, or gravity dam. See also masonry dam. A concrete dam generally requires a sound rock foundation.

Concrete lift: In concrete work, the vertical distance between successive horizontal construction joints.

Concussion: Shock or sharp air waves caused by an explosion or heavy blow.

Condensation: Water vapor changing back into liquid.

Conductor: A substance, body, device, or wire that readily conducts or carries electrical current.

Conduit: A closed channel to convey water through, around, or under a dam. Covered portion of spillway between the gate or crest structure and the terminal structure, where open channel flow and/or pressure flow conditions may exist. Portion of an outlet works between the intake structure and gate chamber and/or the control structure. Conduits are located beneath embankment dams and within concrete dams. Conduits are concrete lined or concrete/steel lined. A pipe, box, or horseshoe structure, or natural channel that is constructed by means of "cut and cover". A conduit can convey water or house other conduits or pipes.

Cone of influence (cone of depression): The depression, roughly conical in shape, produced in the water table by the pumping of water from a well. See circle of influence.

Confined aquifer: An aquifer in which ground water is confined under pressure which is significantly greater than atmospheric pressure. An aquifer that is bound above and below by dense layers of rock and contains water under pressure. See artesian well.

Conifers: Cone-bearing trees or shrubs, mostly evergreens such as pine, cedar, and spruce.

Conjunctive use: The coordinated use of surface water and ground water resources.

Consequences: Potential loss of life or property damage downstream of a dam caused by flood waters released at the dam or by waters released by partial or complete failure of the dam. Includes effects of landslides upstream of the dam on property located around the reservoir. Potential number of lives that would be lost from a dam failure and an uncontrolled reservoir release (considers load, failure modes, and estimated population distribution warning times).

Conservation: Increasing the efficiency of energy use, water use, production, or distribution.

Consolidation: Reduction in particle spacing in a soil, and decrease in water content, resulting from an increase in external pressure.

Constant angle arch dam: An arch dam in which the angle subtended by any horizontal section is constant throughout the whole height of the dam.

Constant radius arch dam: An arch dam in which every horizontal segment or slice of the dam has approximately the same radius of curvature.

Construction joint: Construction joints are purposely placed in concrete to facilitate construction; to reduce initial shrinkage stresses and cracks; to allow time for the installation of embedded metalwork; or to allow for the subsequent placing of other concrete. Bond is required at construction joints regardless of whether or not reinforcement is continuous across the joint. A construction joint allows a reasonable size concrete placement or a point to terminate a placement. The interface between two successive placings of concrete where bond, and not permanent separation, is intended.

Consolidation grouting Strengthening an area of ground by injecting grout.

Consumer surplus: The value of a commodity, good, or opportunity above the cost to the consumer; measured using willingness to pay, as specified in Federal guidelines for water resources planning.

Consumptive use: A use which lessens the amount of water available for another use. Water uses normally associated with man's activities, primarily municipal, industrial, and irrigation uses that deplete water supplies. Water removed from available supplies without direct return to a water resource system, for uses such as manufacturing, agriculture, and food preparation. A nonconsumptive use would be one such as boating or swimming. See beneficial use. Combined amounts of water needed for transpiration by vegetation and for evaporation from adjacent soil, snow, or intercepted precipitation. Also called: crop requirement, crop irrigation requirement, consumptive use requirement. See evapotranspiration.

Consumptive water use: Total amount of water used by vegetation, man's activities, and evaporation of surface water.

Contact grouting: Filling, with cement grout, any voids existing at the contact of two zones of different materials, e.g., between a concrete tunnel lining and the surrounding rock. The grout operation is usually carried out at low pressure.

Contact load: Sediment particles that roll or slide along in almost continuous contact with the streambed.

Containment levee: A dike or embankment to contain stream flow.

Contaminant: A potentially harmful physical, biological, chemical or radiological substance in water. Any physical chemical, biological, or radiological substance or matter that has an adverse effect on air, water, or soil.

Contiguous: Actual contact with; also, near or adjacent to.

Contingencies: Used in appraisal and feasibility estimates to estimate overruns on quantities, changed site conditions, change orders, etc. Contingencies are considered as funds to be used after construction starts and not for design changes or changes in project planning. Appraisal estimates should have 25 percent added and feasibility estimates should have 20 percent added for contingencies.

Contingent valuation: Survey method asking for the maximum values that users would pay for access to a particular activity.

Continuous-flow irrigation: System of irrigation water delivery where each irrigator receives their allotted quantity of water at a continuous rate.

Contour: A line of constant elevation.

Contour ditch: Irrigation ditch laid out approximately on the contour.

Contour farming: System of farming used for erosion control and moisture conservation whereby field operations are performed approximately on the contour. A conservation-based method of farming in which all farming operations (for example, tillage and planting) are performed across (rather than up and down) the slope. Ideally, each crop row is planted at right angles to the ground slope.

Contour flooding: Method of irrigation by flooding from contour ditches.

Contour map: Topographic map that portrays relief or elevation differences by the use of lines (contours) indiciating equal elevation.

Contour plowing: Plowing done in accordance with the natural outline or shape of the land by keeping the furrows or ditches at the same elevation as much as possible to reduce runoff and erosion.

Contour strip farming: A kind of contour farming in which row crops are planted in strips, between alternating strips of close-growing, erosion resistant forage (grass, grain, hay) crops.

Contract rate: The repayment or water service rate set forth in a contract to be paid by a district to the United States.

Contracted weir: The crest and sides of a retangular weir are far enough from the bottom and sides of the channel so that their effect on flow is negligible.

Contraction joint: Contraction joints are placed in concrete to provide for volumetric shrinkage of a monolithic unit or movement between monolithic units. Contraction joints have no bond between the concrete surfaces forming the joint. Except as otherwise provided for dowels, reinforcement is never continuous across a contraction joint. Contraction joints will not transfer moment and will not transfer shear unless keyed.

Control area: Part of a power system, or a combination of systems, to which a common electrical generation allocation scheme is applied.

Control house. See control structure.

Control joint: Control joints are joints placed in concrete to provide for control of initial shrinkage stresses and cracks of monolithic units. Control joints are similar to contraction joints except that reinforcement is always continuous across the joint. Control joints are unbonded joints to provide weak areas for cracking. Control joints will transfer moment, but will not transfer shear unless keyed.

Control structure (control house): Concrete portion of an outlet works, located at the downstream end of the tunnel or conduit, housing the control (regulation) gates. Water regulating structure. A structure on a stream or canal that is used to regulate the flow or stage of the stream.

Conventional tillage: The traditional method of farming in which soil is prepared for planting by completely inverting it with a moldboard plow. Subsequent working of the soil with other implements is usually performed to smooth the soil surface. Bare soil is exposed to the weather for some varying length of time depending on soil and climatic conditions. See tillage.

Conveyance loss (distribution loss): Loss of water from a channel or pipe during conveyance, including losses due to seepage, leakage, evaporation and transpiration by plants growing in or near the channel. Water not available for further use.

Conveyance system efficiency: The ratio of the volume of water delivered to users in proportion to the volume of water introduced into the conveyance system.

Conveyor: A device that transports material by belts, cables, or chains.

Cooperative Agreement: Formal document that states the obligations of Reclamation to one or more other parties. A cooperative agreement provides the authority for the Bureau of Reclamation to issue funding to the other party(ies) listed in the agreement.

Coordinated operation: Generally, the operation of two or more interconnected electrical systems to achieve greater reliability and economy. As applied to hydropower resources, the operation of a group of hydropower plants to obtain optimal power benefits with due consideration to all other uses.

Corduroy: A road made of logs laid crosswise on the ground or on other logs.

Core (impervious core or impervious zone): A zone of low permeability material in an embankment dam. Sometimes referred to as "central core," "inclined core," "puddle clay core," and "rolled clay core." A cylindrical piece of an underground formation cut and raised by a rotary drill with a hollow bit.

Core drill: A rotary drill, usually a diamond drill, equipped with a hollow bit and a core lifter.

Core wall: A wall of substantial thickness built of impervious material, usually of concrete or asphaltic concrete, in the body of an embankment dam to prevent leakage. See membrane.

Correlation: The relation between two or more variables. A statistical test that sets a numerical value to the amount of interdependence.

Corridor: Narrow strip of land reserved for location of transmission lines, pipelines, and service roads.

Corrosion: Wear or dissolving away through chemical action as by rusting, or acids. The gradual decomposition or destruction of a material by chemical action, often due to an electrochemical reaction. Corrosion may be caused by stray current electrolysis, galvanic corrosion caused by dissimilar metals, or differential concentration cells. Corrosion starts at the surface of a material and moves inward.

Corrugated metal pipe (cmp):

Cost-benefit analysis: A quantitative evaluation of the costs which would be incurred versus the overall benefits to society of a proposed action.

Cost effectiveness: Economic efficiency, obtaining the best method for the least amount of money.

Coulees: Small streams or dry streambeds. A deep gulch or ravine, usually dry in the summer.

Coulomb: The quantity of electricity which passes any point in an electric circuit in 1 second when the current is maintained constant at 1 ampere.

Cover crop: A crop that provides temporary protection for delicate seedlings and/or provides a canopy for seasonal soil protection and improvement between normal crop production periods. Except in orchards where permanent vegative cover is maintained, cover crops usually are grown for one year or less. When plowed under and incorporated into the soil, cover crops are also referred to as green manure crops.

Coyote holes: Horizontal tunnels in which explosives are packed for blasting a high rock face.

Crazing: A network pattern of fine cracks in concrete that do not penetrate much below the surface. Crazing cracks are very fine and are barely visible, except when the concrete is drying after it has become wet.

Creep: Slow movement of rock debris or soil usually imperceptible except to observations of long duration. Time- dependent strain or deformation, for example, continuing strain with sustained stress.

Crest: The top surface of the dam. A roadway may be constructed across the crest to permit vehicular traffic or facilitate operation, maintenance, and examination of the dam. Also, the high point of the spillway control section.

Crest elevation (crest of dam, top of dam, dam crest): The elevation of the uppermost surface of a dam, usually a road or walkway, excluding any parapet wall, railing, curb. etc. The crown of the roadway or the level of the walkway which crosses the dam. On embankment dams, the crest of the dam is the top of the embankment, not including camber, crown, or roadway surfacing.

Crest gate (spillway gate): A gate on the crest of a spillway to control the discharge or reservoir water level.

Crest length (length of dam): The distance, measured along the axis or centerline crest of the dam at the top level of the main body of the dam or of the roadway surface on the crest, from abutment contact to abutment contact exclusive of an abutment spillway; provided that, if the spillway lies wholly within the dam and not in any area especially excavated for the spillway, the length includes the spillway.

Crest structure: Portion of spillway between the inlet channel and the chute, tunnel or conduit, which does not contain gates.

Crest width (top thickness): The thickness or width of a dam at the level of the top of dam (excluding corbels or parapets). In general, the term thickness is used for gravity and arch dams, and width is used for other dams.

Crib dam: A gravity dam built up of boxes, cribs, crossed timbers or gabions, filled with earth or rock.

Critical depth: The depth of flow when the Froude number equals one. The particular depth of flow in an open channel with a given discharge at which the specific energy is at a minimum; i.e., the depth at which a given discharge flows in a given channel with a minimum specific energy.

Critical flow: When the Froude number is equal to one, the flow is critical and surface waves remain stationary in the flow. Flow at critical depth.

Critical habitat: Defined in Section 3(5)(A) of the Endangered Species Act (ESA) as:

(1) The specific areas within the geographical area occupied by the species at the time it is listed, on which are found those physical and biological features essential to the conservation of the listed species and which may require special management considerations for protection; and

(2) Specific areas outside the geographical area occupied by a species at the time it is listed upon a determination by the Secretary of the Department of Interior that such areas are essential for the conservation of the species. These areas have been legally designated via Federal Register notices.

Critical slope: The maximum angle with the horizontal at which a sloped bank of soil or rock of given height will stand unsupported.

Crop irrigation requirement: Quantity of water, exclusive of effective precipitation, that is needed for crop production. See irrigation requirement.

Cropping pattern: The acreage distribution of different crops in any one year in a given farm area such as a county, water agency, or farm. Thus, a change in a cropping pattern from one year to the next can occur by changing the relative acreage of existing crops, and/or by introducing new crops, and/or by cropping existing crops.

Crop root zone: The soil depth from which a mature crop extracts most of the water needed for evapotranspiration. The crop root zone is equal to effective rooting depth and is expressed as a depth in inches or feet. This soil depth may be considered as the rooting depth of a subsequent crop, when accounting for soil moisture storage in efficiency calculations.

Crop rotation: A pattern of changing the crops grown in a specific field from year to year in order to control pests and maintain soil fertility. A system of farming in which a regular succession of different crops are planted on the same land area, as opposed to growing the same crop time after time (monoculture).

Crop subsidy: Price support paid by the government to farmers.

Crop water requirement: Crop consumptive use plus the water required to provide the leaching requirements.

Cross-section: Slice of the channel and adjacent valley made perpendicular to the assumed flow. The ground surface and streambed elevations of this slice are used in hydraulic computations. An elevation view of a dam formed by passing a plane through the dam perpendicular to the axis.

Cross-sectional area: Area of a stream, channel or waterway, usually measured perpendicular to the flow.

Crown: The highest point of the interior of a circular conduit, pipe, or tunnel (also referred to as the soffit). The point in an arch dam which generally corresponds with where the height of the dam is a maximum. The elevation of a road center above its sides.

Crushed gravel: Gravel which has been produced by a machine.

Crushed rock: Rock which has been reduced in size by a machine.

Crusher: A machine which reduces rocks to smaller and more uniform sizes.

Cubic feet per second (cfs or ft3/s): A unit of discharge for measurement of a flowing liguid equal to a flow of 1 cubic foot per second (448.8 gallons per minute (gpm), 7.48 gallons per second, or 1.98 acre-feet per day). A rate of streamflow; the volume, in cubic feet, of water passing a reference point in 1 second.

Cubic foot per second (cfs or ft3/s): As a rate of streamflow, a cubic foot of water passing a reference section in one second of time. A measure of a moving volume of water (1 cfs = 0.02832 m3/s).

Cultural patrimony: Ancestral heritage and entitlement.

Cultural resource(s): Any building, site, district, structure, or object significant in history, architecture, archeology, culture, or science. Can include a community's heritage and way of life.

Culvert: A pipe or small bridge for drainage under a road or structure. A conduit for the free passage of surface drainage water under a highway, railroad, canal, or other embankment.

Curb stop (curb cock): A water service shutoff valve located in a water service pipe near the curb and between the water main and the building. This valve is usually operated by a wrench or valve key and is used to start or stop flows in the water service line to a building.

Current (I): The movement of electrons through a conductor, measured in amperes.

Curtain grouting: The process of pressure grouting deep holes under a dam or in an abutment to form a watertight barrier and effectively seal seams, fissures, fault zones, or fill cavities in the foundation or abutment.

Curved gravity dam: A gravity dam which is curved in plan.

Cut: To lower an existing grade or surface level, or an area where this has been done. Gross cut is the total amount of excavation in a road or a road section, without regard to fill requirements. Net cut is the amount of excavated material to be removed from a road section, after completing fills in that section.

Cutting: Excavating, lowering a grade.

Cutoff: An impervious construction by means of which water is prevented from passing through foundation material.

Cutoff trench (keyway): An excavation in the foundation of an embankment dam, usually located upstream of the dam axis or centerline crest which extends to bedrock or to an impervious stratum. The excavation is backfilled with impervious material to form a cutoff and reduce percolation under the dam. See foundation trench.

Cutoff wall: A wall of impervious material (e.g., concrete, asphaltic concrete, timber, steel sheet piling, or impervious grout curtain) located in the foundation beneath a dam and which forms a water barrier and reduces seepage under a dam or spillway.

Cycle: A completed round of regularly recurring events or phenomena.

Cycling: Powerplant operation to meet the intermediated portion of the load (9 to 14 hours per day).

Cyclopean dam: A gravity dam in which the mass masonry consists primarily of large one-man or derrick stone embedded in concrete.

Cylinder gate: A gate that resembles a large barrel, reinforced to withstand external pressure, with no top or bottom. An outlet gate with a vertical hollow cylinder being raised and lowered to expose openings in the outer (usually concrete) wall through which water enters the central area of the gate. Flow in the central area is up or down away from the cylinder.

Dam: A barrier built across a watercourse to impound or divert water. A barrier that obstructs, directs, retards, or stores the flow of water. Usually built across a stream. A structure built to hold back a flow of water. See afterbay dam, ambursen dam, arch dam, buttress dam, check dam, coffer dam, concrete dam, crib dam, detention dam, diversion dam, double curvature arch dam, earth dam, embankment dam, gabion dam, gravity arch dam, gravity dam, hollow gravity dam, hydraulic fill dam, industrial waste dam, masonry dam, mine tailings dam, multiple arch dam, multipurpose dam, overflow dam, precast dam, prestressed dam, regulating dam, rockfill dam, roller-compacted concrete dam, rubble dam, or saddle dam.

Dam failure: Catastrophic type of failure characterized by the sudden, rapid, and uncontrolled release of impounded water. It is recognized that there are lesser degrees of failure and that any malfunction or abnormality outside the design assumptions and parameters which adversely affect a dam's primary function of impounding water could be considered a failure. Such lesser degrees of failure can progressively lead to or heighten the risk of a catastrophic failure. They are, however, normally amenable to corrective action.

Dam failure hydrograph: A flood hydrograph resulting from a dam breach prepared for a specific location downstream from the dam.

Dam foundation: The excavated surface or undisturbed material upon which a dam is placed.

Dam safety deficiency: A physical condition capable of causing the sudden uncontrollable release of reservoir water by partial or complete failure of a dam, appurtenant structure, or facility.

Dam safety issue: Dam safety related issues and concerns are those which, if not adequately addressed could/would:

1 - Lead to a failure or malfunction reulting in an uncontrolled release of stored water that would place the downstream population potentially at risk or;

2 - Compromise the agency's ability to detect developing adverse dam performance and prudently respond to that performance.

Dam tender: The person responsible for the daily or routine operation and maintenance activities of a dam and its appurtenant structures. The dam tender commonly resides at or near the dam.

Damping: Resistance which reduces vibration by energy absorption.

Damping ratio: The ratio of the actual damping to the critical damping, critical damping being the minimum amount of damping which prevents free oscillatory vibration.

Data collection platform: Electronic equipment device that gathers digital and analog data from monitoring devices and remotely relays that data to a central computer for interpretation and archiving.

DataWeb: DataWeb is an electronic presentation of the Bureau of Reclamation's Project Data book and contains historical, statistical, and technical information on the projects of the Bureau of Reclamation.

Datum: Any level surface taken as a plane of reference from which to measure elevations.

Dead capacity (dead storage): The reservoir capacity from which stored water cannot be evacuated by gravity.

Debris fan: Sloping mass of boulders, cobbles, gravel, sand, silt and clay formed by debris flows at the mouth of a tributary.

Debris flow: Flash flood consisting of a mixture of rocks and sediment containing less than 40 percent water, by volume; forms a debris fan.

Decibel: A unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average level at which sound causes pain to humans.

Decision criteria: Set of formal criteria that is used to determine if an event is threatening and what action to implement.

Decision makers: Designated personnel responsible for interpreting data that has been collected and determining if an event is threatening enough to issue an alert or warning.

Decking: Separating charges of explosives by inert material which prevents passing of concussion, and placing a primer in each charge.

Decomposition: Refers to subdividing a failure mode into discrete elements or sequential events so that the failure mode could be better understood, and probabilities can be more reasonably estimated for each step in an event tree.

Deed covenant: A term used in association with the sale of excess lands. In order for an eligible buyer of excess land to receive Reclamation irrigation water on such land, a covenant controlling the sale price of the land must be placed in the deed transferring the land to the buyer.

Deep percolation: The movement of water by gravity downward through the soil profile beyond the root zone; this water is not used by plants. Percolation of irrigation water past the plant root zone to regions of deeper ground-water aquifers.

Deficiency Verification Analysis (DVA): This analysis verifies the existence or non-existence of Safety of Dams (SOD) deficiencies at the dam. Technical memorandums (TM) are prepared which document the analyses used in the investigation of the potential SOD deficiencies. A decision memorandum (DM) is also prepared which recommends if corrective actions should be taken to mitigate the SOD deficiencies. The DVA also includes the establishment of the Design Loading Criteria which include the design earthquake and the inflow design flood (IDF).

Deflagration: To burn with sudden and startling combustion. Describes explosion of black powder, as opposed to the more rapid detonation of dynamite.

Deflation: The force of wind erosion, e.g., blowouts.

Deflect: A decrease in the vertical diameter of a flexible pipe.

Deflection: Upstream or downstream movement of a dam or dike.

Deformation: Change in shape or size.

Degradation: Process wherein the elevation of streambeds, sandbars, and flood plains is lowered by erosion. The opposite of aggradation.

Delay: A device used to obtain detonation of charges at separate times. An electric blasting cap which explodes at a set interval after current is passed through it.

Delay pattern: Order of firing charges obtained by arranging delays to fire separate holes or series of holes at different times.

Delivery: The amount of water delivered to the point of use. The difference between delivery and release is usually the same as consumptive use.

Delta: An alluvial sediment deposit normally formed where a river or stream enters a lake or estuary. Flat land mass of sediment deposit formed at the mouths of streams where they enter larger bodies of water. Sediment deltas are usually triangular in plan view, narrow at the upstream end and relatively wide at the downstream end. The sediment particles deposit because the river velocity and gradient are too low to keep the particles in motion. Active deltas contain diverging multiple channels that continually deposit sediment and migrate back and forth across the delta surface. The sediment particles of the delta deposit are usually well sorted such that the coarser particles (gravel and sand) deposit first at the upstream end, while finer particles (silt and clay) deposit farther downstream. A fan-shaped area at the mouth of a river.

Demand: Rate at which electric energy is used, expressed in kilowatts, whether at a given instant, or averaged over any designated period of time. Maximum water use under a specified condition.

Demand scheduling: Method of irrigation scheduling whereby water is delivered to users as needed and which may vary in flow rate, frequency and duration. Considered a flexible form of scheduling.

Demersal: Fish eggs or organisms that hatch on the bottom of a lake or stream.

Demographics: Relating to the statistical study of human populations.

Dendritic: Channel pattern of streams with tributaries that branch to form a tree-like pattern.

Density: The total mass (solids plus water) per total volume. The weight of soil per unit volume, usually the weight of soil in one cubic foot (unit weight). The weight of a substance per unit of volume of the substance; for example, water has a density of 62.4 pounds per 1 cubic foot. Number per unit area of individuals of any given species at any given time (see population density).

Dentate. See baffle block.

Depletion: To permanently remove water from a system for a specific use. Loss of water from a stream, river, or basin resulting from consumptive use.

Deposition: Material settling out of the water onto the streambed. Occurs when the energy of the flowing water is unable to support the load of suspended sediment. The process of dropping or getting rid of sediments by an erosional agent such as a river or glacier. See sedimentation.

Depth of cutoff: The vertical distance that the cutoff penetrates into the dam foundation.

Derrick: Usually a non-mobile tower equipped with a hoist, but may be used as a synonym for a crane.

Desalinization: The removal of dissolved salts from water by natural means (leaching) or by specific water treatment processes. The process of removing salt from seawater or brackish water.

Desiccant: A drying agent which is capable of removing or absorbing moisture from the atmosphere in a small enclosure.

Desiccate: To dry up; remove moisture from a substance.

Desiccation: The process of drying out. A process used to thoroughly dry air; to remove virtually all moisture from air.

Design basis earthquake (DBE): The earthquake which the structure is required to safely withstand with repairable damage. Those systems and components important to safety must remain functional and/or operable. For design purposes, the intended use of this earthquake loading is for economic design of structures or components whose damage or failure would not lead to catastrophic loss. For most usage in Reclamation, the DBE is defined to have a 90% probability of nonoccurrence in a 50-year-exposure period, which is equivalent to a recurrence interval of 474 years. Economic considerations for specific projects may lead to consideration of other values.

Design response spectra: Smooth, broad-banded spectra appropriate for specifying the level of seismic design force, or displacement, for earthquake-resistant design purposes.

Design summary: A document that summarizes the designers' development of the design that results in the specifications. It may include a section on the Designers' Operating Criteria.

Design water level: The maximum water elevation, including the flood surcharge, that a dam is designed to be able to withstand.

Design wind: The most severe wind that is reasonbly possible at a particular reservoir for generating wind setup and runup. The determination will generally include the results of meteorologic studies which combine wind velocity, duration, direction, and seasonal distribution characteristics in a realistic manner.

Designated floodway: The channel of a water course and those portions of the adjoining flood plain required to provide for the passage of a selected flood with a small increase in flood stage above that of natural conditions.

Designated frequency flood: A 100-year flood is often considered in the design of diversion dams and for diversion-during-construction requirements. Service spillways, stilling basins, and some outlet works components may also be designed to pass certain level of floods designated by a return period. The return period should be thought as the chance that such a flood will be equaled or exceeded in any one year. For example, the 100-year flood is the flow level with a 0.01 annual exceedance probability, or there is 1 chance in 100 that this flood flow level will be equaled or exceeded in any given year.

Designers' Operating Criteria (DOC): Detailed operating criteria which stress the designer's intended use and operation of equipment and structures in the interest of safe, proper, and efficient use of the facilities.

Desorption: The release or removal of an adsorbed material from the surface of a solid adsorbent.

Destratification: The development of vertical mixing within a lake or reservoir to eliminate (either totally or partially) separate layers of temperature, plant, or animal life. This vertical mixing can be caused by mechanical means (pumps) or through the use of forced air diffusers which release air into the lower layers of the reservoir.

Detention dam: A dam built to store streamflow or surface runoff, and to control the release of such stored water.

Detonating cord: Round, flexible textile, or plastic cord with a center core of high explosive. A common use is to tie in shots using nonelectric delays (MS connectors) when the use of electric caps would be hazardous or undesirable.

Detonation: Practically instantaneous decomposition or combustion of an unstable compound, with tremendous increase in volume.

Detonator: A device to start an explosion, as a fuse or cap.

Detritus: Loose material that results directly from disintegration.

Dew: Moisture in the air that condenses on solid surfaces when the air is saturated with water vapor.

Dew point: The temperature to which air with a given quantity of water vapor must be cooled to cause condensation of the vapor in the air.

Dewatering As opposed to unwatering, dewatering is the removal and control of ground water from pores or other open spaces in soil or rock formations to the extent that allows construction activities to proceed as intended, including the relief of ground water pressure. Removing water by pumping, drainage, or evaporation. The removal of ground water and seepage from below the surface of the ground or other surfaces through the use of deep wells and wellpoints.

Diagnostic tool: Any artifact which, because of form, shape, or function, provides chronological or manufacturing information.

Diaphragm. See membrane.

Diaphragm pump: A pump that moves water by reciprocating motion of a diaphragm in a chamber having inlet and outlet check valves.

Diaphragm-type earthfill: An embankment dam which is constructed mostly of pervious material and having a diaphragm of impermeable material which forms a water barrier. The diaphragm which forms the water barrier may consist of earth, Portland cement concrete, bituminous concrete, or other material, and may occupy a position within the embankment or on the upstream face.

Diaphragm wall: A sheet, thin zone, or facing made of a relatively impervious material such as concrete, steel, wood, plastic, etc. Also see core wall.

Diatom: Single-celled or colonial algae whose cell walls are made of silica.

Differential head (unbalanced head): The condition in which the water pressure on the upstream and downstream sides of an object differ.

Digitizer: Device that converts analog information into a digital format; for flat graphic material, such as maps, a digitizer can either be flatbed or scanning.

Dike: A low embankment, usually constructed to close up low areas of the reservoir rim and thus limit the extent of the reservoir. Embankment for restraining a river or a stream. Embankments which contain water within a given course. Usually applied to dams built to protect land from flooding. See saddle dam.

Dilatancy: The expansion of cohesionless soils when subject to shear deformation.

Dilution: Reduction of the concentration of a substance in air or water.

Dip: The angle that a stratum or any planar feature makes with the horizontal, measured perpendicular to the strike and in the vertical plane. The slope of layers of soil or rock.

Direct current (DC): Electrical current flowing in one direction only and essentially free from pulsation. See alternating current.

Direct runoff: Water that flows over the ground surface or through the ground directly into streams, rivers, or lakes.

Direct shear test: A shear test in which soil or rock under an applied normal load is stressed to failure by moving one section of the soil container (shear box) relative to the other section.

Disaster: An event that demands a crisis response beyond the scope of any single line agency or service (e.g., beyond the scope of just the police department, fire department, etc.) and that presents a threat to a community or larger area. A disaster requires resources beyond what are available locally.

Discharge: Volume of water that passes a given point within a given period of time. See flow. Any spilling, leaking, pumping, pouring, emitting, emptying, or dumping not including permitted activities in compliance with section 402 of the Clean Water Act.

Discharge capacity: The maximum amount of water that can safely released from a given waterway.

Dispersion: Distortion of the shape of a seismic-wave train because of variation of velocity with frequency.

Displacement pump: Reciprocating type of pump where a piston draws water into a closed chamber and then expels it under pressure.

Dissolve: To enter into a solution.

Dissolved oxygen (DO): Amount of free oxygen found in water; perhaps the most commonly employed measurement of water quality. Low DO levels adversely affect fish and other aquatic life. The ideal dissolved oxygen for fish life is between 7 and 9 mg/L; most fish cannot survive when the DO level falls below 3 mg/L.

Distribution efficiency: Measure of the uniformity of irrigation water distribution over a field.

Distribution line: Low voltage electric power line, usually 69 kilovolts or less.

Distribution system: System of ditches, or conduits and their appurtenances, which conveys irrigation water from the main canal to the farm units.

District: An entity that has a contract with the Bureau of Reclamation for the delivery of irrigation water. Such entities include, but are not limited to: canal companies, conservancy districts, ditch companies, irrigation and drainage districts, irrigation companies, irrigation districts, reclamation districts, service districts, storage districts, water districts, and water users associations.

Ditch: Generally, a long narrow excavation. Constructed open channel for conducting water.

Diversion: A process which, having return flow and consumptive use elements, turns water from a given path. Removal of water from its natural channel for human use. Use of part of a stream flow as a water supply. Channel constructed across the slope for the purpose of intercepting surface runoff, changing the accustomed course of all or part of a stream. A structural conveyance (or ditch) constructed across a slope to intercept runoff flowing down a hillside, and divert it to some convenient discharge point.

Diversion capacity: The flow which can be passed through the canal headworks at a dam under normal head.

Diversion channel (canal or tunnel): A waterway used to divert water from its natural course. The term generally applies to a temporary arrangement (e.g., to bypass water around a damsite during construction). Channel is normally used instead of canal when the waterway is short. Occasionally the term is applied to a permanent arrangement (diversion canal, diversion tunnel, diversion aqueducts).

Diversion dam: A dam built to divert water from a waterway or stream into a different watercourse.

Diversion inlet: A conduit or tunnel upstream from an intake structure. Diversion inlet may be integral with the outlet works or be part of a separate conveyance structure that will only be used during construction.

Divert: To direct a flow away from its natural course.

Divide: A ridge or high area of land that separates one drainage basin from another.

Dome: A structure curved both in the lengthwise and crosswise direction.

Domestic water use (residential water use): Water for household purposes, such as drinking, food preparation, bathing, washing clothes and dishes, flushing toilets, and water lawns and gardens.

Double curvature arch dam: An arch dam which is curved in plan and elevation, with undercutting of the heel and in most instances, a downstream overhang near the crest. An arch dam which is curved vertically as well as horizontally.

Downstream face: The inclined surface of a dam away from the reservoir. See face.

Draft: The act of drawing or removing water from a reservoir.

Drainage: Process of removing surface or subsurface water from a soil or area. A technique to improve the productivity of some agricultural land by removing excess water from the soil; surface drainage is accomplished with open ditches; subsurface drainage uses porous conduits (drain tile) buried beneath the soil surface.

Drainage area: The area which drains to a particular point on a river or stream. The drainage area of a stream at a specified location is that area, measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream above the specified point.

Drainage basin: All of the area drained by a river system. The drainage basin is a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water. The area of land that drains its water into a river.

Drainage blanket: A layer of pervious material placed directly over the foundation material to facilitate drainage of the foundation and/or embankment.

Drainage curtain: A line of vertical wells or boreholes to facilitate drainage of the foundation and abutments and to reduce water pressure (also called drainage wells or relief wells).

Drainage layer: A layer of pervious material in an earthfill dam to relieve pore pressures or to facilitate drainage of the fill.

Drainage system: Collection of surface and/or subsurface drains, together with structures and pumps, used to remove surface or ground water.

Drainage well (relief well): Vertical wells or boreholes downstream of, or in downstream shoulder of, an embankment dam to collect and control seepage through or under the dam and so reduce water pressure. A line of such wells forms a drainage curtain.

Draw: A small valley or gully.

Drawdown: Lowering of a reservoir's water level; process of depleting a reservoir or ground water storage. The drop in the water table or level of water in the ground when water is being pumped from a well. Vertical distance the free water surface elevation is lowered or the reduction of the pressure head due to the removal of free water. The difference between a water level and a lower water level in a reservoir within a particular time. The amount of water used from a reservoir.

Dredge: To dig under water. A machine that digs under water.

Drift: Food organisms, including algae, plankton, and even larval fish, dislodged and moved by river current. A small, nearly horizontal tunnel.

Drip irrigation (trickle): An irrigation method in which water is delivered to or near each plant in small-diameter plastic tubing. The water is then discharged at a rate less than the soil infiltration capacity through pores, perforations, or small emitters on the tubing. The tubing may be laid on the soil surface, be shallowly buried, or be supported above the surface (as on grape trellises).

Drop structure: A structure that conveys water to a lower elevation and dissipates the excess energy resulting from the drop.

Drought: Climatic condition in which there is insufficient soil moisture available for normal vegetative growth. A prolonged period of below-average precipitation.

Drum gate: A movable crest gate in the form of a sector of a cylinder hinged at the centerline. The arc face effects a seal with edge of a recess and the gate is operated by a reservoir pressure. A type of spillway gate consisting of a long hollow drum hinged at either the upstream or downstream end. In the type called the Reclamation Drum the hinge is on the upstream edge. The drum may be held in its raised position by the water pressure in a flotation chamber beneath the drum. Design permits overtopping.

Dry well: A deep hole, covered, and usually lined or filled with rocks, that holds drainage water until it soaks into the ground.

Ductility: The property of a material that allows it to be formed into thin sections without breaking.

Dumped: A method of compacting soil by dumping the soil into place with no compactive effort.

Dynamic compaction: A method of compacting soil by dropping a heavy weight onto loose soil.

Dynamic equilibrium: Condition achieved when the average sand load transported by flowing water is in balance with the sand load being supplied by tributaries.

Dynamic pressure: When a pump is operating, the vertical distance from a reference point (such as a pump centerline) to the hydraulic grade line.

Early warning system (EWS): A designed system that will ensure timely recognition of a threatening event and provide a reliable and timely warning and evacuation of the population at risk from dangerous flooding associated with large operational releases or dam failure. The designed system must address the five components of detection, decision making, notification, warning, and evacuation.

Early warning system feasibility: Study of the feasibility of installing some level of Early Warning System to meet the target times for alerts and warnings.

Earth dam (earthfill dam): An embankment dam in which more than 50 percent of the total volume is formed of compacted earth material generally smaller than 3-inch size. Seepage through the dam is controlled by the designed use of upstream blankets and/or internal cores constructed using compacted soil of very low permeability.

Earth lining: Compacted layer of earth on surface of canal or other excavation.

Earthquake: A sudden motion or trembling in the earth caused by the abrupt release of accumulated stress along a fault. See design basis earthquake (DBE), maximum credible earthquake (MCE), maximum design earthquake (MDE), operating basis earthquake (OBE), and random earthquake.

Earthwork: Any one or combination of the operations involved in altering or movement of earth.

Easement: The right to use land owned by another for some specific purpose.

Ecology: Branch of biological science which deals with relationships between living organisms and their environments.

Economic analysis: A procedure that includes both tangible and intangible factors to evaluate various alternatives.

Ecosystem: Complex system composed of a community of people, animals, and plants as well as the chemical and physical environments.

Eddy: Circular current of water moving against the main current. See recirculation zone.

Effective peak ground acceleration: That acceleration which is most closely related to structural response and to damage potential of an earthquake.

Efficiency: Ratio of useful energy output to total energy input, usually expressed as percent. Effective operation as measured by a comparison of production with cost.

Effluent: Partially or completely treated wastewater flowing out of a treatment facility, reservoir, or basin.

Effluent limitation: The maximum amount of a specific substance or characteristic that can be present in effluent discharge without violating water quality standards in receiving waters.

Elbow: A pipe fitting having two openings which causes a run of pipe to change direction 90 degrees.

Electric power system: Physically connected electric generating, transmission, and distribution facilities operated as a unit under one control.

Electrical conductivity: A measure of the salt content of water.

Electrical demand: Energy requirement placed upon a utility's generation at a given instant or averaged over any designated period of time; expressed in kilowatts.

Electromotive force (EMF): The electrical pressure available to cause a flow of current when an electrical circuit is closed. See voltage.

Elevation: The height of a point above a plane of reference. Generally refers to the height above sea level. See datum.

Elongate: An increase of the vertical diameter of a flexible pipe.

Embankment: An earth structure the top of which is higher than the adjoining surface. A shaped earth or rockfill dam. Fill material, usually earth or rock, placed with sloping sides and with a length greater than its height. An embankment is generally higher than a dike.

Embankment dam or fill dam: Any dam constructed of excavated natural materials. See diaphragm-type earthfill, earth dam or earthfill dam, homogeneous earthfill dam, hydraulic fill dam, rockfill dam, rolled fill dam, or zoned earthfill. See also berm and fill.

Emergency: A condition of a serious nature which develops unexpectedly and endangers the structural integrity of a dam or endangers downstream property and human life and requires immediate action. An event that demands a crisis response beyond the scope of any single line agency or service (e.g., beyond the scope of just the police department, fire department, etc.) and that presents a threat to a community or larger area. An emergency is an event that can be controlled within the scope of local capabilities.

Emergency action plan (EAP): Reclamation procedures for dam operating personnel to follow during emergency situations or unusual occurrences at a given dam to reduce potential for property damage and loss of life and to provide proper notification to downstream authorities. A formal plan of procedures designed to minimize an emergency situation or unusual occurrence at a given dam or reservoir.

Emergency broadcast system: A federally established network of commercial radio stations that voluntarily provide official emergency instructions or directions to the public during an emergency. Priorities for Emergency Broadcast System activation and use are: first, Federal Government; second, local government; and third, State government.

Emergency classification levels: A phased system in which dam operating organizations classify dam safety emergency incidents into response levels according to how severe they are at the time of observation and as to time of occurrence. Declaring a response level is followed by providing appropriate notifications to downstream local authorities.

Response Level I - The first, and least serious, of three response levels that the dam operating organization will declare after analyzing a potentially threatening event. An event in this alert category may be perceived as an emergency or may be of general interest to the public, but does NOT pose a hazard, either at the dam or to downstream populations at risk when observed.

Response Level II - The second of three response levels the dam operating organization will declare after analyzing a threatening event. Declaring Response Level II means that emergency conditions are such that populations at risk should prepare to leave predetermined inundation areas for higher ground and safe shelter. Declaration of Response Level II means that an event has occurred or is likely to occur that will actually threaten the structure and/or areas downstream from the dam if the event continues and/or intensifies.

Response Level III - The third, and most serious, response level the dam operating organization will declare after analyzing threatening events. Declaring Response Level III indicates that life-threatening floodwaters, as a result of high operational releases or dam failure, present imminent danger to the public located downstream from a dam. Declaration of Response Level III should prompt local officials to immediately evacuate populations at risk.

Emergency evacuation zones: Geographical areas delineated in inundation areas downstream from a dam (hazard generator) that define the potential area of impact and allow prioritizing evacuation activities based on proximity of the populations at risk to the hazard in terms of distance and floodwave travel times.

Emergency Evacuation Zone I - The emergency evacuation zone immediately below a dam and located on both sides of the river or stream. Generally, it is recommended that this zone extend to a distance equivalent to a combination of floodwave travel time of 0-15 minutes and/or a warning time of 0-4 hours, whichever is most conservative. May be called immediate response zone, evacuation zone I, evacuation zone A, or other appropriate name.

Emergency Evacuation Zone II - The second emergency evacuation zone, beyond emergency evacuation zone I and also located on both sides of the river or stream. Generally, it is recommended that this zone extend to a distance using a combination of floodwave travel times of between 15-90 minutes and/or warning times of between 4-6 hours, whichever is most conservative. May be called protective action zone, evacuation zone II, evacuation zone B, or other appropriate name.

Emergency Evacuation Zone III - The outermost emergency evacuation zone, extending beyond emergency evacuation zone II and also located on both sides of the river or stream. Its furthest point is that beyond which emergency planning and evacuation for a dam failure inundation would not be required under most conditions; where the floodwave would be attenuated; and beyond which the potential negative impacts on humans would be virtually eliminated. May be called precautionary zone, evacuation zone III, evacuation zone C, or other appropriate name.

Emergency gate: A standby or auxiliary gate used when the normal means of water control is not available. The first gate in a series of flow controls, remaining open while downstream gates or valves are operating. See guard gate.

Emergency management: The system by which mitigation, preparedness, response, and recovery activities are undertaken to save lives and protect property from all hazards.

Emergency management agency: Any State or local agency responsible for emergency operations, planning, mitigation, preparedness, response, and recovery for all hazards. Names of emergency management agencies may vary, but could include: Division of Emergency Management, Comprehensive Emergency Management, Disaster Emergency Services, Civil Defense Agency, Emergency and Disaster Services, etc.

Emergency on-scene coordinator (Reclamation OSC): At all times, there must be at least one employee either on the premises or on call (i.e., available to respond to an emergency by reaching the facility within a short period of time) with the responsibility for coordinating all emergency response measures. This emergency coordinator must be thoroughly familiar with all aspects of the facility's contingency plan, all operations and activities at the facility, the location and characteristics of hazardous substances handled, the location of all records within the facility, and the facility layout. In addition, this person must have the authority to commit the resources needed to carry out the contingency plan. See on-scene coordinator.

Emergency operations center: The location of facility where responsible officials gather during an emergency to direct and coordinate emergency operations, to communicate with other jurisdictions and with field emergency forces, and to formulate protective action decisions and recommendations during an emergency.

Emergency operations plan (State and local): A plan, usually developed in accord with guidance contained in the Guide for the Development of State and Local Emergency Operations Plans, Civil Preparedness Guide 1-8, September 1990, and other similar guides. The emergency operations plan clearly and concisely describes a jurisdiction's emergency organization, its means of coordination with other jurisdictions, and its approach to protecting people and property from disasters and emergencies caused by any of the hazards to which the community is particularly vulnerable. It assigns functional responsibilities to the elements of the emergency organization, and details tasks to be carried out at times and places projected as accurately as permitted by the nature of each situation addressed. Emergency operations plans are multi-hazard, functional plans that treat emergency management activities generically. They have a basic section that provides generally applicable information without reference to any particular hazard. They also address the unique aspects of individual disasters in hazard-specific appendixes.

Emergency Preparedness Plan (EPP): Predecessor to the term Emergency Action Plan (EAP).

Emergency program manager: The individual responsible on a day-to-day basis for a jurisdiction's effort to develop a capability for coordinated response to and recovery from the effects of emergencies and large-scale disasters. This official may be called the local emergency manager, civil defense director, disaster preparedness coordinator, or other similar title; the duties may vary from jurisdiction to jurisdiction.

Emergency reserve fund: Money reserved or required by contract to be reserved by an operating entity for use in emergency situations involving facilities under the entity's jurisdiction.

Emergency spillway: A spillway which provides for additional safety should emergencies not contemplated by normal design assumptions be encountered, i.e., inoperable outlet works, spillway gates, or spillway structure problems. The crest is usually set at maximum water surface. A spillway that is designed to provide additional protection against overtopping of dams and is intended for use under extreme conditions such as misoperation or malfunction of the service spillway or other emergency conditions.

Endangered species: A species or subspecies whose survival is in danger of extinction throughout all or a significant portion of its range.

Energy: Force or action of doing work. Measured in terms of the work it is capable of doing; electric energy, the electric capacity generated and/or delivered over time, is usually measured in kilowatt hours (kWh).

Energy dissipator: A device constructed in a waterway to reduce the kinetic energy of fast flowing water. See stilling basin.

Enhancement:Improvement of a facility beyond its originally designed purpose or condition.

Enhancement flow: Improved flows that result in better stream conditions for aquatic, terrestrial, and other resources.

Ensign valve: Earliest type of Reclamation needle valve (named after Reclamation's chief electrical engineer), operated by water pressure from the forebay, and mounted on the upstream side of the dam.

Entrain: To trap bubbles in water either mechanically through turbulence or chemically through a reaction.

Entrainment: Process by which aquatic organisms, suspended in water, are pulled through a pump or other device.

Environment: All biological, chemical, social, and physical factors to which organisms are exposed. The surroundings that affect the growth and development of an organism.

Environmental analysis: Systematic process for consideration of environment factors in land management actions.

Environmental assessment (EA): A NEPA compliance document used to determine if an action would have a significant effect on the human environment. If not, a finding of no significant impact (FONSI) is written. If so, an environmental impact statement (EIS) is written.

Environmental impact statement (EIS): A NEPA compliance document used to evaluate a range of alternatives when solving the problem would have a significant effect on the human environment. The EIS is more than a document, it is a formal analysis process which mandates public comment periods. An EIS covers purpose and need, alternatives, existing conditions, environmental consequences, and consultation and coordination.

Environmental Protection Agency (EPA): The Environmental Protection Agency's mission is to protect human health and to safeguard the natural environment.

Eolian: Windblown. See aeolian.

Ephemeral creek: A creek or stream that flows briefly only in direct response to precipitation and whose channel is above the water table. See intermittent stream.

Epicenter: Focal point on the earth's surface directly above the origin of a seismic disturbance. Point on the Earth's surface vertically above the earthquake focus or hypocenter.

Epifauna: Animals which live on the benthos.

Epilimnion: The upper, or top, layer of a lake or reservoir with essentially uniform warmer temperatures. The upper layer of water in a thermally stratified lake or reservoir. This layer consists of the warmest water and has a fairly constant temperature. The layer is readily mixed by wind action. See stratification.

Erode: To wear away or remove the land surface by wind, water, or other agents.

Erosion: A gradual wearing away of soil or rock by running water, waves, or wind. Concrete surface disturbance caused by cavitation, abrasion from moving particles in water, impact of pedestrian or vehicular traffic, or impact of icefloes. Surface displacement of soil caused by weathering, dissolution, abrasion, or other transporting. The gradual wearing away of material as a result of abrasive action.

Escapement: Unharvested spawning stocks that return to the streams.

Escarpment: A cliff or steep slope that separates two level or gently sloping areas.

Essential element: A structural or geologic feature or an equipment item whose failure under the particular loading condition or set of circumstances being considered would create a dam safety deficiency.

Esthetics: An emotional judgment about what is beautiful or pleasing.

Estuary: The area of mixing of fresh water and salt water where a river flows into an ocean.

Eutrophic: Nutrient enrichment of a body of water that contains more organic matter than existing biological oxidization processes can consume. A body of water which has become, either naturally or by pollution, rich in nutrients and often seasonally deficient in dissolved oxygen. Reservoirs and lakes which are rich in nutrients and very productive in terms of aquatic animal and plant life.

Eutrophication: A process where more organic matter is produced than existing biological oxidization processes can consume. The increase in the nutrient levels of a lake or other body of water; this usually causes an increase in the growth of aquatic animal and plant life.

Evacuation warning: A public warning message that local officials would issue to evacuate flood inundation areas.

Evaluation report (Report for Examination No. 1, first formal examination): The report of the initial SEED onsite examination.

Evaporation: Water vapor losses from water surfaces, sprinkler irrigation, and other related factors. Loss of water to the atmosphere. The process by which water is changed from a liquid into a vapor. Water from land areas, bodies of water, and all other moist surfaces is absorbed into the atmosphere as a vapor.

Evapotranspiration: The quantity of water transpired by plants or evaporated from adjacent soil surfaces in a specific time period. Usually expressed in depth of water per unit area. The combined processes of evaporation and transpiration. It can be defined as the sum of water used by vegetation and water lost by evaporation. A collective term that includes water discharged to the atmosphere as a result of evaporation from the soil and surface water and as a result of plant transpiration.

Examination report: A written report that documents the condition of the facility during the examination, operation and maintenance activities accomplished since the last examination, and recommendations necessary for the continued safe and efficient operation of the facility.

Excavation: The action or process of excavating (to dig or remove earth). See common excavation, rock excavation, and unclassified excavation.

Excess capacity: Power generation capacity available on a short- term basis that exceeds the firm energy on a long-term contract offered to an electricity customer.

Excess land: Irrigable land, other than exempt land, owned by any landowner in excess of the maximum ownership entitlement under applicable provision of Reclamation law.

Exclusive flood control capacity: The reservoir capacity assigned to the sole purpose of regulating flood inflows to reduce flood damage downstream. In some instances, the top of exclusive flood control capacity is above the maximum controllable water surface elevation. See flood control capacity.

Exempt land: Irrigation land in a district to which the acreage limitation and pricing provisions of Reclamation law do not apply.

Existence value: Value people place on simply knowing an area or feature continues to exist in a particular condition.

Existing ground: The earth's surface as it is prior to any work. See original ground surface.

Exit channel. See outlet channel.

Exit conference: A discussion following a facility review examination involving examination team members and interested representatives of the water uses, project, and region. The topics of discussion include the overall condition of the facility, any recommendations made as a result of the examination, and any other pertinent topics.

Exotic species: A non-native species that is introduced into an area.

Expansion joint: A separation between adjoining parts of a concrete structure which is provided to allow small relative movements, such as those caused by temperature changes, to occur independently. A flexible filler is provided in the joint, reinforcement does not pass through the joint. A joint that permits pipe to move as a result of expansion.

Exploit: Excavate in such a manner as to utilize material in a particular vein or layer, and waste or avoid surrounding material.

Extirpated species: A species which has become extinct in a given area.

Extrados: The curved upstream surface of horizontal arch elements in an arch dam. See intrados.

Extrapolation: Estimation of unknown values by extending or projecting from known values.

Face: Exposed surface of dam materials (earth, rockfill, or concrete), upstream and downstream. The external surface which limits the structure, see neatlines. The more or less vertical surface of rock exposed by blasting or excavating. The cutting end of a drill hole.

Facilities: Structures associated with Reclamation irrigation projects, municipal and industrial water systems, power generation facilities, including all storage, conveyance, distribution, and drainage systems.

Facing: With reference to a wall or concrete dam, a coating of a different material, masonry or brick, for architectural or protection purposes, e.g., stonework facing, brickwork facing. With reference to an embankment dam, an impervious coating or face on the upstream slope of the dam.

Factor of safety: The ratio of the ultimate strength of the material to the allowable or working stress.

Fahrenheit (F): Unit of temperature. Degrees Fahrenheit equals (9/5)x(degrees Celsius)+(32).

Failure: An incident resulting in the uncontrolled release of water from a dam. Destroyed and made useless, ceases to function as a dam. More severe and hazardous than a breach. See dam failure.

Failure potential assessment: A judgment of the potential for failure of an essential element within the expected life of the project. Five terms are used to describe the assessment: negligible, low, moderate, high, and urgent. A rating of negligible reflects the judgment that failure of the essential element is regarded as very unlikely; low reflects the judgment that failure is unlikely; moderate reflects the judgment that failure is possible and further data collection and/or analyses may be required; high reflects the judgment that failure is very probable; urgent reflects the judgment that failure is imminent.

Fall: The amount of slope given to horizontal runs of pipe.

Fallow: Land plowed and tilled and left unplanted.

False set. See horsehead.

Farm loss (water): Water delivered to a farm which is not made available to the crop to be irrigated.

Fatal flaw: Any problem, lack, or conflict (real or perceived) that will destroy a solution or process. A negative effect that cannot be offset by any degree of benefits from other factors.

Fatality rate: A multiplication factor based on the estimated severity of the flood, potential warning times, and the judgement related to the understanding of the flood severity.

Fault: A fracture or fracture zone in the earth's crust along which there has been displacement of the two sides relative to one another or in rock along which the adjacent rock surfaces are differentially displaced. A shear with significant continuity that can be correlated between observation points. See active fault.

Faulting: The movement which produces relative displacement along a fracture in rock.

Fauna: All animals associated with a given habitat, country, area, or period.

Feasibility: A determination that something can be done. A feasibility report is required in some planning processes to examine the situation and determine if a workable solution can be developed and implemented.

Feasibility estimate: An estimate used for determining the economic feasibility of a project, the probable sequence and cost for construction of a project, and as a guide in the choice between alternative locations or plans.

Feather: To blend the edge of new material smoothly into the old surface.

Federal Emergency Management Agency (FEMA): Federal agency responsible for enforcing the legislation for disaster and emergency planning and response. See FEMA.

Federal Energy Regulatory Commission (FERC): Agency primarily responsible for ensuring adequate energy supplies at just and reasonable rates and providing regulatory incentives for increased productivity, efficiency, and competition. See FERC.

Fetch: The straight line distance across a body of water subject to wind forces. The distance which wind passes over water. The fetch is one of the factors used in calculating wave heights in a reservoir. The area in which waves are generated by a wind having a fairly constant direction and speed.

Field capacity (field moisture capacity): Depth of water retained in the soil after ample irrigation or heavy rain when the rate of downward movement has substantially decreased, usually one to three days after irrigation or rain, expressed as a depth of water in inches or feet.

Field tile: Short lengths of clay pipe that are installed as subsurface drains.

Fill: Manmade deposits of natural soils or the process of the depositing. An earth or broken rock structure or embankment. Soil or loose rock used to raise a grade. Soil that has no value except as bulk.

Filter (filter zone): One or more layers of granular material which is incorporated in an embankment dam and is graded (either naturally or by selection) to allow seepage through or within the layers while preventing the migration of material from adjacent zones.

Filter cake (mud cake): A deposit of mud on the walls of a drill hole.

Financial analysis: Procedure that considers only tangible factors when evaluating various alternatives.

Finding of no significant impact (FONSI): A NEPA compliance document which affirms that an environmental assessment found that alternatives were evaluated and a proposed action would have no significant impact on the human environment.

Fines: Portion of a soil finer that a No. 200 U.S. Standard sieve. Clay or silt particles in soil.

Finger drains: A series of parallel drains of narrow width (instead of a continuous drainage blanket) draining to the downstream toe of the embankment dam.

Finished grade: The elevation or surface of the earth after all earthwork has been completed (also finish grade). The final grade required by specifications.

Firm energy (power): Non-interruptible energy and power guaranteed by the supplier to be available at all times, except for uncontrollable circumstances.

Firm yield: The maximum quantity of water that can be guaranteed with some specified degree of confidence during a specific critical period. The critical period is that period in a sequential record that requires the largest volume from storage to provide a specified yield.

Fish & Wildlife Service (FWS): The U.S. Fish and Wildlife Service is the principal Federal agency responsible for conserving, protecting and enhancing fish, wildlife and plants and their habitats for the continuing benefit of the American people. FWS is an agency within the U.S. Department of the Interior.

Fish ladder (fishway): An inclined trough which carries water from above to below a dam so that fish can easily swim upstream. There are various types, some with baffles to reduce the velocity of the water and some consisting of a series of boxes with water spilling down from one to the next.

Fish weir: A type of fish ladder.

Fishery: The aquatic region in which a certain species of fish lives.

Fishing: The operation of recovering an object left or dropped in a drill hole.

Fixed amount-frequency scheduling: Method of irrigation scheduling that involves water delivery at a fixed rate or a fixed volume and at constant intervals. Examples include rotation and continuous flow methods. Considered a rigid form of scheduling.

Fixed cone valve (Howell Bunger valve): A cylinder gate mounted with the axis horizontal.

Fixed-wheel gate (fixed-roller gate, fixed-axle gate): A gate consisting of a rectangular leaf mounted on wheels, particularly suited for high head situations. A gate having wheels or rollers mounted on the end posts of the gate. The wheels bear against rails fixed in side grooves or gate guides.

Flail: A hammer hinged to an axle so that is can be used to break or crush material.

Flange: A ridge that prevents a sliding motion. A rib or rim for strength or for attachments. A rim or collar attached to one end of a pipe to give support.

Flap gate: A gate hinged along one edge, usually either the top or bottom edge. Examples of bottom-hinged flap gates are tilting gates and fish belly gates - so called from their shape in cross section.

Flashboards: Temporary barriers, consisting of either timber, concrete or steel, anchored to the crest of a spillway as a means of increasing the reservoir storage. Flashboards can be removed, lowered, or carried away at the time of flooding either by a tripping device or by deliberate failure of the flashboards or their supports. Structural members of timber, concrete, or steel placed in channels or on the crest of a spillway to raise the reservoir water level but that may be quickly removed in the event of a flood.

Flash flood: A flood which follows within a few hours of heavy or excessive rainfall.

Flash flood warning: Flash flooding has been reported or is imminent.

Flash flood watch: Flash flooding is possible within the designated watch area.

Flat slab or slab and buttress dam: A buttress dam with buttresses which support the flat slab of reinforced concrete which forms the upstream face.

Fleet angle: The angle between the position of a rope or cable at the extreme end wrap on a drum and a line drawn perpendicular to the axis of the drum. The fleet angle is used to indicate how effective or efficient the rope or cable is for raising a load.

Flexible pipe: Pipe designed to transmit the backfill load to the soil at the sides of the pipe. Flexible pipe must be supported on both the bottom and on the sides of the pipe.

Fling: Refers to a near field long period pulse from a strong ground motion resulting in a unidirectional ground heave after rupture. Great kinetic energy may be associated with a fling and is important in near field records.

Flip bucket: An energy dissipator located at the downstream end of a spillway and shaped so that water flowing at a high velocity is deflected upwards in a trajectory away from the foundation of the spillway. See stilling basin.

Floatable days: The number of days during the recreation season on which it is safe to allow floating activities on recreation facilities.

Floatable flows: River flows which make rafting and other floating recreation possible.

Floc: Loose, open-structured mass formed in a suspension by the aggregation of minute particles. Clumps of bacteria and particulate impurities that have come together and formed a cluster. Found in flocculation tanks and settling or sedimentation basins. Clumps of impurities removed from water during the purification process; formed when alum is added to impure water.

Flocculation: A step in water filtration in which alum is added to cause particles to clump together.

Flood: A temporary rise in water levels resulting in inundation of areas not normally covered by water. May be expressed in terms of probability of exceedance per year such as 1-percent chance flood or expressed as a fraction of the probable maximum flood or other reference flood.

Flood boundary: Line drawn or outer edge of colored (inundation) area on an inundation map to show the limit of flooding.

Flood control capacity: Reservoir capacity assigned to the sole purpose of regulating flood inflows to reduce flood damage downstream. See exclusive flood control capacity.

Flood control pool (flood pool): Reservoir volume above active conservation capacity and joint use capacity that is reserved for flood runoff and then evacuated as soon as possible to keep that volume in readiness for the next flood.

Flood frequency: Refers to the probability (in percent) that a flood will occur in a given year.

Flood gate: A gate to control flood releases from a reservoir.

Flood hydrograph: A graphical representation of the flood discharge with respect to time for a particular point on a stream or river.

Flood irrigation: Method of irrigating where water is applied from field ditches onto land which has no guide preparation such as furrows, borders or corrugations.

Flood plain: Nearly level land, susceptible to floods, that forms the bottom of a valley. An area, adjoining a body of water or natural stream, that has been or may be covered by floodwater.

Flood pool index: Computed as the ratio of the flood control pool depth to the depth below the pool, multiplied by the percent of time the reservoir water surface will be within the flood control pool.

Flood routing: A process of determining progressively over time the amplitude of a flood wave as it moves past a dam or downstream to successive points along a river or stream.

Flood stage: An established gage height within a given river reach above which a rise in water surface level is defined as a flood.

Flood severity: Qualitative description of how severe a possible flood could be (High, Medium, Low) depending on failure modes (including rate of failure), flood velocity, channel width, magnitude of damage potential, rate of rise for flood waters, etc. High severity would be associated with structures being swept clean from their foundations. Low severity would indicate that a slow, gradual rise of flood waters is anticipated.

Flood severity understanding: Understanding as to what degree flooding might affect the downstream population. The judgement on flood severity understanding is based on type of loading and is described as either vague or precise.

Flora: All plant life associated with a given habitat, country, or period. Bacteria are considered flora.

Flow: Volume of water that passes a given point within a given period of time. See base flow, discharge, enhancement flow, instream flow requirements, interstitial flow, minimum flow, peak flow, and return flow.

Flow augmentation: The release of water stored in a reservoir or other impoundment to increase the natural flow of a stream.

Flowage: Water that floods onto adjacent land.

Flowage easement: The right or easement to overflow, submerge, or flood certain lands; a right to prohibit building on certain floodways.

Flowline: The path that a particle of water follows in its course of seepage under laminar flow conditions.

Flownet: A graphical representation of flowlines and equipotential (piezometric) lines used in the study of seepage phenomena.

Fluctuating flows: Water released from a dam that varies in volume with time.

Fluctuating zone: Area of a sandbar or vegetation zone that is within the range of fluctuating flow.

Flume: Shaped, open-channel flow sections that force flow to accelerate. Acceleration is produced by converging the sidewalls, raising the bottom, or a combination of both. An artificial channel, often elevated above ground, used to carry fast flowing water. See long- throated flume, Parshall flume, and short-throated flume.

Fluvial: Pertains to streams and stream processes.

Flushing: A method used to clean water distribution lines by passing a large amount of water through the system.

Fluvial: Of or pertaining to rivers; growing or living in a stream or river; produced by the action of a stream or river.

Fly ash: A by-product of coal-fired powerplants which reacts with water and the free lime in cement while generating only half the heat of an equal amount of cement.

Food chain: A succession of organisms in a community in which food energy is transferred from one organism to another as each consumes a lower member and, in turn, is consumed by a higher member.

Foot: Twelve inches. One of a number of projections on a cylindrical drum of a tamping roller.

Foot-Pound: Unit of work equal to the force in pounds multiplied by the distance in feet through which it acts. When a 1 pound force is exerted through a 1 foot distance, 1 foot pound of work is done.

Footing: A sill under a foundation. Ground, in relation to its load bearing and friction qualities. Portion of the foundation of a structure that transmits loads directly to the soil.

Forage: Vegetation used for animal consumption.

Forage fish: Generally, small fish that produce prolifically and are consumed by predators.

Forb: A weed or a broad-leafed plant.

Forced outage: Unscheduled shut down of a generating unit or other facility for emergency or other unforeseen reasons.

Ford: A place where a road crosses a stream under water.

Forebay (headrace): Impoundment immediately upstream from a dam or hydroelectric plant intake structure. The term is applicable to all types of hydroelectric developments (storage, run-of-river, and pumped- storage).

Forepole: A plank driven ahead of a tunnel face to support the roof or wall during excavation. See horsehead.

Foreshore: That part of the shore between the ordinary high-and low-watermarks and generally crossed by the tide each day.

Forest Service: The mission of the USDA Forest Service is to sustain the health, diversity, and productivity of the Nation's forests and grasslands to meet the needs of present and future generations. The Forest Service is an agency within the U.S. Department of Agriculture.

Formation: Any sedimentary, igneous, or metamorphic material represented as a unit in geology; generally called rock, but not necessarily meeting the definition of rock.

Formulation process: First phase performed by the Early Warning System design team, which includes an Early Warning System reliability, local capabilities assessment, and conceptual level designs for an Early Warning System.

Fossorial insects: Insects that live in the soil.

Foundation: Lower part of a structure which transmits the load to the earth. The excavated surface upon which a dam is placed.

Foundation drains: Tile or pipe for collecting seepage within a foundation.

Foundation material (foundation soil): The upper part of the earth mass carrying the load of the structure.

Foundation surface: The surface of the upper part of the earth mass carrying the load of the structure.

Foundation trench: A trench built at and into the foundation of a dam and filled with clay or other impermeable substances to prevent water from seeping beneath the dam. See cutoff trench.

Freeboard: The difference in elevation between the maximum water surface in the reservoir and the dam crest. The vertical distance between a stated water level and the top of a dam, without camber. Thus "net freeboard," "dry freeboard," or "flood freeboard" is the vertical distance between the maximum water surface and the top of the dam. "Gross freeboard" or "total freeboard" is the vertical distance between the normal water surface and the top of the dam. That part of the "gross freeboard" attributable to the depth of flood surcharge is sometimes referred to as the "wet freeboard," but this term is not recommended as it is preferable that freeboard be stated with reference to the top of dam.

Freezeout: Deeply frozen over for long periods of time.

Freeze-thaw damage: Damage to concrete caused by extreme temperature variations as noted by random pattern cracking. Damage is accelerated by the presence of water and commonly more severe on the south-facing side of structures.

French drain: A covered ditch containing a layer of fitted or loose stone or other pervious material.

Frequency demand scheduling: Method of irrigation scheduling similar to demand scheduling, but typically involves a fixed duration of the delivery, such as 24 hours. This method is considered flexible, although somewhat less so than demand scheduling from the water users' perspective.

Freshwater: Water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.

Friction: Resistance to motion when one body is sliding or tending to slide over another.

Front end loader: A tractor loader that both digs and dumps in front.

Frost action: Freezing and thawing of moisture in materials and the resultant effects on these materials and on structures of which they are a part of with which they are in contact.

Frost heave: The raising of a surface due to the accumulation of ice in the underlying soil or rock.

Frost line: The greatest depth to which ground may be expected to freeze.

Froude number: The ratio of inertial forces to gravitational forces in flow. It is also the ratio of the flow velocity to the velocity of a small gravity wave in the flow. When the Froude number is less than one, the flow is tranquil. When the Froude number is greater than one, the flow is rapid. When the Froude number is equal to one, the flow is critical.

Fry: Life stage of fish between the egg and fingerling stages. Depending on the species of fish, fry can measure from a few millimeters to a few centimeters.

Fuel replacement energy: Electric energy generated at a hydroelectric plant as a substitute for energy which would have been generated by a thermal electric plant.

Fulcrum: A pivot for a lever.

Full hydraulic capacity: The designed capacity of a pipe or conduit.

Full irrigation service land: Irrigable land now receiving, or to receive, its sole and generally adequate water supply through facilities which have been or are to be constructed by, rehabilitated by, or replaced by the Bureau of Reclamation.

Full pool: Volume of water in a reservoir at normal water surface. The reservoir level that would be attained when the reservoir is fully utilized for all project purposes, including flood control.

Furrow: A natural or man-made narrow depression in the earth's surface. A narrow trenchlike plowed depression in the earth surface to keep surface water away from the slopes of cuts.

Fuse: A thin core of black powder surrounded by wrappings, which when lit at one end, will burn to the other at a fixed speed. A string-like core of PETN, a high explosive, contained within a waterproof reinforced sheath. "Primacord" is the best known brand.

Fuse plug spillway: A form of auxiliary spillway consisting of a low embankment designed to be overtopped and washed away during an exceptionally large flood.

Future without: What would occur if no action were taken. The future without taking any action to solve the problem. See baseline condition.

Gabion: Wire basket, filled with stones, used to stabilize banks of a water course and to enhance habitat.

Gabion dam: Special name given to a crib dam when built with gabions. See dam.

Gage height (G.H.): The water surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeably with the more general term "stage," although gage height is more appropriate when used with a reading on a gage. See gauge height.

Gaging station: A particular site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained.

Gaining stream: Stream or reach that receives water from the zone of saturation.

Gallery: A passageway within the body of a dam, its foundation, or abutments. Used for inspection, foundation grouting, and/or drainage.

Gallon: A unit of measure equal to four quarts or 128 fluid ounces.

Galvanize: To coat a metal (especially iron or steel) with zinc. Galvanization is the process of coating a metal with zinc.

Gametes: Eggs or sperm.

Gantry: An overhead structure that supports machines or operating parts, such as a gantry crane.

Gantry crane: A fixed or traveling, bent-supported crane for handling heavy equipment.

Gaseous supersaturation: Condition of higher levels of dissolved gases in water due to entrainment, pressure increases, or heating.

Gate: A device that controls the flow in a conduit, pipe, or tunnel without obstructing any portion of the waterway when in the fully open position. Structure or device for controlling the rate of flow into or from a canal or ditch. See bear-trap gate, bulkhead gate, clam shell gate, coaster gate, crest gate, cylinder gate, drum gate, emergency gate, fixed-wheel gate, flap gate, flood gate, guard gate, high-pressure gate, jet-flow gate, outlet gate, paradox gate, radial gate, regulating gate, ring gate, ring- follower gate, ring seal gate, roller gate, skimmer gate, slide gate, sluice gate, stoney gate, tainter gate, tractor gate, vertical lift gate, or wicket gate.

Gate chamber: A chamber in which a guard gate in a pressurized outlet works or both the guard and regulating gates in a free-flow outlet works is located. Concrete portion of an outlet works containing gates between upstream and downstream conduits and/or tunnels.

Gate hanger: A device used to maintain a set gate opening.

Gate valve: A valve with a circular-shaped closing element that fits securely over an opening through which water flows. A valve that utilizes a disc moving at a right angle to the flow to regulate the rate of flow. When a gate valve is fully opened, there is no obstruction to the flow.

Gate structure: Portion of spillway between inlet channel and chute, tunnel or conduit, which contains gates, such as radial gates.

Gated pipe: Portable pipe with small gates installed along one side for distributing irrigation water to corrugations or furrows.

Gated spillway: Overflow section of dam restricted by use of gates that can be operated to control releases from the reservoir to ensure the safety of the dam.

Gauge (gage): Device for registering water level, discharge, velocity, pressure, etc. Thickness of wire or sheet metal. A number that defines the thickness of the sheet used to make steel pipe. The larger the number, the thinner the pipe wall.

Gauge height: Elevation of water surface measured by a gauge. See gage height.

Gauge pressure: Absolute pressure minus atmospheric pressure. The pressure within a closed container as measured with a gauge. Sometimes referred to as relative pressure.

Gauging station: Specific location on a stream where systematic observations of hydrologic data are obtained through mechanical or electrical means. See gauge.

Generation: The energy generated in kWh (kilowatt-hours) represents gross generation. It consists of the total generation minus station use. Process of producing electric energy by transforming other forms of energy. Amount of electric energy produced, expressed in kilowatt-hours.

Generator: Machine that converts mechanical energy into electrical energy.

Geohydrology: Geological study of the character, source, and mode of ground water.

Geology: The science that deals with the physical history of the earth, the rocks of which it is comprised, and the physical changes which the earth has undergone or is undergoing.

Geomorphic: Of or relating to the form or shape of the earth.

Geomorphology: Geological study of the configuration, characteristics, origin, and evolution of land forms and earth features.

Geophysics: Refers to the physics of the earth, e.g., seismology, oceanography, volcanology, geomagnetism, etc.

Geotextiles: Any fabric or textile (natural or synthetic) used as an engineering material in conjunction with soil, foundations, or rock. Geotextiles provide the following uses: drainage, filtration, separation of materials, reinforcement, moisture barriers, and erosion protection.

Giant salvinia (Salvinia molesta): An aquatic fern prohibited in the United States by Federal law. An invasive, rapidly growing plant that covers the surfaces of lakes and streams forming floating mats that shade and crowd out important native plants. Thick mats reduce oxygen content, degrading water quality for fish and other organisms, impeding boating, fishing and swimming, and clogging water intakes for irrigation and electrical generation. The plant spreads aggressively by fragmenting and has oblong floating leaves, 1/2 to 1 1/2 inches long. Young plants have smaller leaves that lie flat on the water surface. As the plant matures and aggregates into mats, leaves fold and compress into upright chains. For more information visit the Giant Salvinia website.

Gigawatt (gw): Unit of power equal to 1 billion watts.

Gigawatt-hour (GWh): One billion watthours of electrical energy.

Glacial moraine: A mass of loose rock, soil, and earth deposited by the edge of a glacier.

Glacial striations: Lines carved into rock by overriding ice, showing the direction of glacial movement.

Glacial till: Material deposited by glaciation, usually composed of a wide range of particle sizes, which has not been subjected to the sorting action of water.

Glacier: A large mass of ice formed on land by the compacting and recrystallization of snow; glaciers survive from year to year, and creep downslope or outward due to the stress of their own weight.

Global positioning systems (GPS): Space-based radio positioning systems that provide 24-hour, three-dimensional position, velocity, and time information to suitably equipped users anywhere on or near the surface of the Earth.

Goggle valve. See ring follower gate.

Gradation: Proportion of material of each grain-size present in a given soil.

Grade (pitch): The elevation of a surface or a surface slope. The elevation of the invert of the bottom of a pipeline, canal, culvert, or conduit. The fall (slope) of a line of pipe in reference to a horizontal plane. The inclination or slope of a pipeline, conduit, stream channel, or natural ground surface; usually expressed in terms of the ratio or percentage of number of units of vertical rise or fall per unit of horizontal distance (rise over run).

Graded stream: Streams that receive and carry away equal amounts of sediment.

Grader: A machine with a centrally located blade that can be angled to cast to either side, with independent hoist control on each side.

Grade stake: A stake indicating the amount of cut or fill required to bring the ground to a specified level.

Gradient: General slope or rate of change in vertical elevation per unit of horizontal distance of water surface of a flowing stream. Slope along a specific route, as of a road surface, channel or pipe.

Grapple: A clamshell-type bucket having three or more jaws.

Gravel: Loose rounded fragments of rock that will pass a 3-inch sieve and be retained on a No. 4 U.S. Standard sieve (3/16 inch).

Gravelfill: Gravel used to fill holes or spaces.

Gravel blanket: Thin layer of gravel spread over an area either of natural ground, excavated surface, or embankment.

Gravel surfacing: Layer of gravel spread over an area intended for vehicular or personnel traffic, such as roads, parking lots and sidewalks.

Gravity arch dam: A dam designed to combine load resisting features of both a gravity and arch type dam.

Gravity dam: A dam constructed of concrete and/or masonry which relies on its weight and internal strength for stability. Gravity dams are generally used where the foundation is rock and earthfill in proper quality and quantity is not available. See arch-gravity dam, crib dam, curved gravity dam, cyclopean dam, and hollow gravity dam.

Gravity irrigation: Irrigation method that applies irrigation water to fields by letting it flow from a higher level supply canal through ditches or furrows to fields at a lower level.

Grizzly (grizzlie): A coarse screen used to remove oversize pieces from earth or blasted rock. A gate or closure on a chute.

Groin: The contact between the upstream or downstream face of a dam and the abutments.

Gross crop value: This value is the sum of annual receipts from sale of crops produced. Production of crops, such as pasture and hay which normally are consumed on the farm by livestock, shall be converted to cash market values and included with crop sales. Total market value of all crop production from irrigated lands before deducting costs of production. Unit prices represent the weighted average prices received by farmers for the part of the crop that is sold. Production and price information are obtained from reports of farmers, project-operating personnel, local agricultural specialists, and State-Federal agricultural statisticians.

Gross generation: Total amount of electrical energy produced by a generating station or stations, measured at generator terminals.

Ground-fault circuit interrupter (GFCI): An electrical device designed to protect people (not equipment) from electrical shock. The GFCI is a very sensitive device that can detect ground leakage currents as low as 5 milliamperes. The GFCI can be provided as part of a receptacle or as part of a circuit breaker. When the GFCI detects a ground leakage current, it either deenergizes the receptacle or trips the circuit breaker. Whether the GRCI is part of a receptacle or part of a circuit breaker, it will have a TEST button. Pressing the TEST button will operate the GFCI device and deenergize the circuit. The receptacle will have a RESET button and the circuit breaker is manually reset by hand.

Ground motion: A general term including all aspects of ground motion, namely particle acceleration, velocity, or displacement, from an earthquake or other energy source.

Ground motion parameters: Numerical values representing vibratory ground motion, such as particle acceleration, velocity, and displacement, frequency content, predominant period, spectral intensity, and duration.

Ground water: Water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper level of the saturated zone is called the water table. Water stored underground in rock crevices and in the pores of geologic materials that make up the earth's crust. That part of the subsurface water which is in the zone of saturation; phreatic water. Water found underground in porous rock strata and soils, as in a spring. Water under ground, such as in wells, springs and aquifers. Generally, all subsurface water as distinct from surface water; specifically, that part of the subsurface water in the saturated zone where the water is under pressure greater than atmospheric.

Ground water mining (overdraft): Pumping of ground water for irrigation or other uses, at rates faster than the rate at which the ground water is being recharged.

Ground water recharge: The flow to ground water storage from precipitation, inflitration from streams, and other sources of water.

Ground water table: The upper boundary of ground water where water pressure is equal to atmospheric pressure, i.e., water level in a bore hole after equilibrium when ground water can freely enter the hole from the sides and bottom.

Grout: A fluid mixture of cement and water or sand, cement, and water used to seal joints and cracks in a rock foundation. A fluid material that is injected into soil, rock, concrete, or other construction material to seal openings and to lower the permeability and/or provide additional structural strength. There are four major types of grouting materials: chemical, cement, clay, and bitumen.

Grout blanket: An area of the foundation systematically grouted to a uniform depth.

Grout cap: A concrete pad or wall constructed to facilitate subsequent pressure grouting of the grout curtain beneath the grout cap.

Grout curtain (grout cutoff): A vertical zone, usually thin, in the foundation into which grout is injected to reduce seepage beneath a dam.

Grouting: Filling cracks and crevices with a cement mixture.

Growing season: The period, often the frost-free period, during which the climate is such that crops can be produced.

Grubbing: Removal of stumps, roots, and vegetable matter from the ground surface after clearing and prior to excavation.

Guard gate (emergency): The first gate in a series of flow controls, remaining open while downstream gates or valves are operating. A gate, usually located between the emergency and regulating gates, used in the closed position to permit servicing of the downstream regulating gate(s) or valve(s) or the downstream conduit. See emergency gate.

Gullying: Small-scale stream erosion.

Habitat: The area or type of environment in which a plant or animal normally lives or occurs.

Halophytic: Salt-loving. Plants that thrive in soils that contain salt and/or sodium. A plant that grows in salty or alkaline soil.

Hanger: A support for pipe.

Hardness: A characteristic of water determined by the levels of calcium and magnesium.

Hardpan: A hard, impervious layer, composed chiefly of clay, that is cemented by relatively insoluble materials, that does not become plastic when mixed with water, and definitely limits the downward movement of water and roots. A cemented or compacted layer of soil near the surface that is essentially impermeable to water. A hard, tight soil. A hard layer that may form just below plow depth on cultivated land.

Harrow: An agricultural tool that loosens and works the ground surface.

Harvest: In a recreational fishery, refers to numbers of fish that are caught and kept. See catch.

Hatchery: A place for hatching fish eggs.

Haul distance: The distance measured along the center line or most direct practical route between the center of the mass of excavation and the center of mass of the fill as finally placed. It is the distance material is moved. Average haul is the average distance a grading material is moved from cut to fill. See overhaul.

Haunches (haunch): The outside areas between the springline and the bottom of a pipe. In pipe, the sides of the lower third of the circumference.

Hazard: Something (e.g., a dam) that creates the potential for adverse consequences such as loss of life (LOL), property damage, and adverse social and environmental impacts. From a dam safety perspective, impacts may be from floodwaters released from dam structures or waters released by partial or complete failure of the dam. In this case, impacts would be to a defined area downstream. Impacts may also be to an area upstream of the dam from effects of backwater flooding or effects of landslides around the reservoir perimeter.

Hazard classification: The rating for a dam based on the potential consequences of failure. The rating is based on potential loss of life and damage to property that failure of the dam cause. Such classification is related to the amount of development downstream of a dam. Hazard classification is not associated with the existing condition of a dam and/or its appurtenant structures or the anticipated performance or operation of a dam. Rather, hazard classification is a statement of the most realistic adverse impact on human life and downstream developments should a designated dam fail. Hazard classification is used as a tool for prioritizing program activities, allocating resources for accomplishment of objectives, and scheduling safety of dams reassessments. See high hazard, low hazard, and significant hazard.

Hazardous materials: Materials that pose the potential for grave, immediate, future, and genetic injury and illness when handled without proper equipment and precautions. Such materials may be toxic, flammable, explosive, corrosive, combinations of these, or otherwise injurious to life and health. Besides being potentially injurious to the discoverer of the materials, toxic materials may be transported to co-workers, children, or pets from shoes or clothing.

Hazardous substances: Element or compound other than oil which when discharged into the environment, in any quantity, presents an imminent or substantial threat to public health or welfare.

Hazardous Waste Indicators: Indicators that may signal the presence of hazardous materials include stressed vegetation or unusual lack of vegetation; dead or sick domestic stock, wildlife, or birds; fish kills or otherwise unexplained stream sterility or diminished species and numbers of flora and fauna; unusual coloration or discoloration of the land surface; and acrid or other chemical odors. However, some of the indicators may be ambiguous.

Hazardous waste sites: Any spill, authorized or unauthorized dumping, abandoned, or inactive waste disposal sites containing or suspected of containing hazardous materials. Materials may be in drums, cylinders, canisters, sacks, or may be uncontained in piles of solids, pools of liquids, abandoned tailings, ponds, or as clouds of gasses.

Head: Differential of pressure causing flow in a fluid system, usually expressed in terms of the height of a liquid column that pressure will support. The difference in number of feet between two water surface elevations. Height of water above a specified point. The back-pressure against a pump.

Header: See manifold.

Heading: In a tunnel, a digging face and its work area.

Head loss: The energy lost from a flowing fluid due to friction, transitions, bends, etc.

Headrace. See forebay.

Headwall: An upstream wall.

Headwater: The source and upper part of a stream; water upstream of a dam or powerhouse.

Heap: The soil carried above the sides of a body or bucket.

Heave: The upward movement of land surfaces or structures due to subsurface expansion of soil or rock, or vertical faulting of rock. Upward movement of soil caused by expansion or displacement resulting from phenomena such as moisture absorption, removal of overburden, driving of piles, frost action, and loading of an adjacent area.

Hectare: A measure of area in the metric system similar to an acre. One hectare is equal to 10,000 square meters and 2.4711 acres.

Heel of dam: The junction of the upstream face of a concrete gravity dam or arch dam with the ground surface. For an embankment dam, the junction is referred to as the upstream toe of dam.

Height of dam. See hydraulic height or structural height.

Helical: Spiral.

Herbicide: A compound, usually a man-made organic chemical, used to kill or control plant growth.

Herbivore: Animal that feeds on plants.

Herpetofauna: Reptiles and amphibians.

Hertz (Hz): The number of complete electromagnetic cycles or waves in one second of an electrical or electronic circuit. A unit of frequency equal to one cycle per second.

High dam: A dam over 300 feet high. See low dam and medium-height dam.

High hazard: A downstream hazard classification for dams in which more than 6 lives would be in jeopardy and excessive economic loss (urban area including extensive community, industry, agriculture, or outstanding natural resources) would occur as a direct result of dam failure. This classification also applies to structures other than dams.

High-pressure gate: A gate consisting of a rectangular leaf encased in a body and bonnet and equipped with a hydraulic hoist for moving the gate leaf.

Hoe: A shovel that digs by pulling a broom-and-stick-mounted bucket toward itself.

Hoist: The mechanism by which a bucket or blade is lifted, or the process of lifting it.

Hollow gravity dam (cellular gravity dam): A dam which has the outward appearance of a gravity dam but is of hollow construction. A dam constructed of concrete and/or masonry on the outside but having a hollow interior and relying on its weight for stability.

Hollow-jet valve: A valve having a closing member that moves upstream to shut off the flow. The hollow-jet valve discharges a hollow or annular jet dispersed over a wide area.

Homogeneous earthfill dam: An embankment dam construction throughout of more or less uniform earth materials, except for possible inclusion of internal drains or blanket drains. Used to differentiate it from a zoned earthfill dam. An embankment type dam constructed of only one type of material.

Hopper: A storage bin or a funnel that is loaded from the top, and discharges through a door or chute in the bottom.

Horsehead (false set): A temporary support for forepoles used in tunneling soft ground.

Horsepower (hp): A measurement of power that includes the factors of force and speed. The force required to lift 33,000 pounds one foot in one minute.

Hub: The enlarged end of a bell and spigot cast-iron pipe.

Human environment: Natural and physical environment and the relationship of people with that environment, including all combinations of physical, biological, cultural, social, and economic factors in a given area.

Humus: Decayed organic matter. A dark fluffy swamp soil composed chiefly of decayed vegetation. A brown or black material formed by the partial decomposition of vegetable or animal matter. The organic portion of the soil remaining after prolonged microbial decomposition. See peat.

Hydraulic: Powered by water. Having to do with water in motion.

Hydraulic efficiency: Efficiency of a pump or turbine to impart energy to or extract energy from water. The ability of hydraulic structure or element to conduct water with minimum energy loss.

Hydraulic fill: Fill material that is transported and deposited using water.

Hydraulic fill dam: An embankment dam constructed of materials, often dredged, which are conveyed and/or placed by suspension in flowing water.

Hydraulic fill structure: A dam or impoundment made of hydraulic fill.

Hydraulic gradient: The slope of the surface of open or underground water.

Hydraulic height: Height to which the water rises behind the dam, and is the difference between the lowest point in the original streambed at the axis or the centerline crest of the dam, or the invert of the lowest outlet works, whichever is lower, and the maximum controllable water surface. See structural height.

Hydraulics: Having to do with the mechanical properties of water in motion and the application of these properties in engineering.

Hydraucone: A draft tube in which the emerging water impinges on a plate.

Hydric: Characterized by, or thriving in, an abundance of moisture.

Hydroelectric plant: Electric powerplant using falling water as its motive force. A power plant that produces electricity from the power of rushing water turning turbine-generators.

Hydroelectric power: Electrical energy produced by flowing water.

Hydrogeochemistry: Chemistry of ground water and surface water.

Hydrogeologic conditions: Conditions stemming from the interaction of ground water and the surrounding soil and rock.

Hydrogeologic cycle: The natural process of recycling water from the atmosphere down to (and through) the earth and back to the atmosphere again.

Hydrogeologist: A person who studies and works with ground water.

Hydrogeology: The geology of ground water, with particular emphasis on the chemistry and movement of water.

Hydrograph: A graphical representation of the stage or discharge as a function of time at a particular point on a watercourse; a time-discharge curve of the unsteady flow of water. A graph showing for a given point on a stream, river, or conduit, the discharge, stage, velocity, available power, rate of runoff, or other property of water with respect to time. This can be measured or modeled.

Hydrography: Scientific study of physical aspects of all waters on the Earth's surface. Water features in 7.5-minute quads include lakes, shorelines, and drainage routing.

Hydrologic cycle: Cycle of water movement from atmosphere to Earth by precipitation and its return to the atmosphere by interception, evaporation, runoff, infiltration, percolation, storage, and transpiration. The natural recycling process powered by the sun that causes water to evaporate into the atmosphere, condense and return to earth as precipitation.

Hydrologic unit code: An eight-digit number used to identify a geographic area representing part or all of a surface drainage basin or distinct hydrologic feature.

Hydrology: Scientific study of water in nature: its properties, distribution, and behavior. The science that treats the occurrence, circulation properties, and distribution of the waters of the earth and their reaction to the environment. Science dealing with the properties, distribution and flow of water on or in the earth.

Hydromet: A system of data collection platforms that gathers hydrometeorological data, and transmits that data via GOES Satelite to a computer downlink.

Hydrometer: A device for measuring the specific gravity of fluids.

Hydrometeorological Report (HMR): A series of hydrometeorological reports published by the National Weather Service (NWS) addressing meteorological issues related mainly to developing estimates of probable maximum precipitation used in the determination of the probable maximum flood for design of water control structures.

Hydrophilic: Having a strong affinity (liking) for water. The opposite of hydrophobic.

Hydrophobic: Having a strong aversion (dislike) for water. The opposite of hydrophilic.

Hydrostatic: Relating to pressure or equilibrium of fluids.

Hyperconcentrated flow: Moving mixture of sediment and water between 40 and 80 percent water by volume.

Hypocenter: The point or focus within the earth which is the center of an earthquake and the origin of its elastic waves. The location within the Earth where the sudden release of energy is initiated. The focus of an earthquake.

Hypolimnetic: Pertaining to the lower, colder portion of a lake or reservoir which is separated from the upper, warmer portion (epilimnion) by the thermocline.

Hypolimnion: The lower, or bottom, layer of a lake or reservoir with essentially uniform colder temperatures. The lowest layer in a thermally stratified lake or reservoir. This layer consists of colder, more dense water, has a constant temperature and no mixing occurs. See stratification.

Hyporheic zone: Ground water habitats created by the movement of river water from the active channel to areas to the side and beneath the active channel. Uniquely adapted organisms that can provide food for fish live in the ground water habitat.

Hypsography: Elevation measurement system based on a sea level datum.

Ichthyology: The scientific study of fish.

Igneous: Rock formed by molten magma.

Impeller: A rotary pump member using centrifugal force to discharge a fluid into outlet passages. A rotating set of vanes in a pump designed to pump or lift water. See centrifugal pump.

Impermeable: Having a texture that does not permit water to move through quickly.

Impervious: Not permeable; not allowing liquid to pass through. Resistant to movement of water.

Impervious core. See core.

Impervious material: Relatively waterproof soils. Clay, for example, since water will percolate through clay at about one millionth of the rate at which it will pass through gravel.

Impervious zone. See core.

Impinge: To collide or strike.

Implementation process: Procurement, installation, and implementation of all the components of an Early Warning System; includes inspecting and exercising the system.

Impoundment: Body of water created by a dam.

Improvement: Structural measures for the betterment, modernization, or enhancement of an existing facility or system to improve the social, economic, and environmental benefits of the project.

Inactive capacity (inactive storage): The reservoir capacity exclusive of and above the dead capacity from which the stored water is normally not available because of operating agreements or physical restrictions. Under abnormal conditions, such as a shortage of water or a requirement for structural repairs, water may be evacuated from this space. The inactive capacity extends from the top of inactive capacity to the top of dead capacity.

Incident command system: The combination of facilities, equipment, personnel, procedures, and communications operating within a common organizational structure to effectively respond to an emergency or disaster.

Incised stream: A stream that has cut its channel into the bed of the valley through degradation.

Incremental loss of life: A measurement for assessing damage after a dam fails. The difference between the projected loss of life had the dam not been built and the projected loss of life with the dam in place.

Independent power producers: Non-utility owned electric resources.

Indicator: Organism, species, or community which indicates certain environmental conditions.

Indigenous: Native to a given area.

Industrial waste dam: An embankment dam, usually built in stages, to create storage for the disposal of waste products from an industrial process. The waste products are conveyed as fine material suspended in water to the reservoir impounded by the embankment. The embankment may be built of conventional materials but sometimes incorporates suitable waste products.

Infiltration: Flow of a liquid into a substance through pores of small openings. See permeability.

Infiltration rate: The rate of water entry into the soil expressed as a depth of water per unit of time in inches per hour or feet per day. The infiltration rate changes with time during irrigation.

Inflow: Water that flows into a body of water. The amount of water entering a reservoir expressed in acre-feet per day or cubic feet per second.

Inflow design flood (IDF): The flood used to design and/or modify a specific dam and its appurtenant works; particularly for sizing the spillway and outlet works, and for determining surcharge storage and height of dam requirements. The flood used for design of a safe structure. It may be the probable maximum flood (PMF), but in sparsely developed areas where judgment indicates minimal property damage and no probable loss of life, the IDF may be less than the PMF.

Initial responders (first responders): Individuals who are likely to witness or discover a hazardous substance release and who have been trained to initiate an emergency response sequence by notifying the proper authorities of the release.

Initial SEED examination: The first formal onsite examination. See evaluation report and SEED examination.

Inlet channel (inlet structure): Concrete lined portion of spillway between approach channel and gate or crest structure.

Inlet structure. See inlet channel.

In situ: In place, the original location, in the natural environment.

Institutionalized populations People in schools, hospitals, nursing homes, prisons, federal buildings, or other facilities that require special care or consideration during emergencies by virtue of their dependency on others for appropriate protection.

Instream flow requirements: Amount of water flowing through a defined stream channel needed to sustain instream values, e.g. flows designated for fish and wildlife.

Instream uses: Water uses that can be carried out without removing the water from its source, as in navigation and recreation.

Instrumentation: Any device used to monitor the performance of the structure during its construction and throughout its useful life. An arrangement of devices installed into or near dams (i.e., piezometers, inclinometer, strain gages, measurement points, etc.) and used to evaluate the structural behavior and performance parameters of the structure.

Intake: Any structure through which water can be drawn into a waterway.

Intake structure: Concrete portion of an outlet works, including trashracks and/or fish screens, upstream from the tunnel or conduit portions. The entrance to an outlet works.

Intangible factors: Factors that affect a decision, but that cannot be expressed in monetary terms. Examples include employee morale, safety, system reliability, environmental effects, and politics.

Integrated early warning system: A flood warning system tied into adjacent systems so that multiple users can access the basin data and monitor the event.

Intensity scale: An arbitrary scale to describe the degree of shaking at a particular place. The scale is not based on measurement, but on assessment by an experienced observer. A numerical index describing the effects of an earthquake on mankind, on structures built by mankind, and on the earth's surface. The scale in common use in the U. S. today is the Modified Mercalli Scale of 1931 with grades indicated by Roman numerals from I to XII. For more information, visit the U.S. Geological Survey National Earthquake Information Center.

Intercepting Drain: Curtain drain.

Interconnected system: System consisting of two or more individual power systems normally operating with connecting tie lines.

Interflow: Lateral movement of water in the upper layer of soil.

Intermediate SEED examination: An onsite examination performed approximately every 3 years, or more frequently if conditions dictate, to evaluate the operation, performance, and existing condition of all features.

Intermittent stream (seasonal stream): A stream which flows part of the time, usually after rainstorm, during wet weather, or for only part of the year. Also referred to as an ephemeral stream. Stream on or in contact with the ground water table that flows only at certain times of the year when the ground water table is high.

Internal drainage: Movement of water down through soil to porous aquifers or to surface outlets at lower elevations. Drainage within a basin that has no outlet.

Internal erosion: The formation of voids within soil or soft rock caused by the mechanical or chemical removal of material by seepage. See erosion.

Internal vibration: A method of consolidating soil in which vibrators are used within a thoroughly wetted soil mass to consolidate the soil to the desired density.

Interruptible demands: Demands that, by contract, can be interrupted if the supplying system lacks capacity.

Interstitial flow: Portion of surface water that infiltrates the streambed and moves through pores in subsurface.

Intrados: The curved downstream surface of horizontal arch elements of an arch dam. See extrados.

Intrusion: A feature (landform, vegetation, or structure) that is generally considered out of context because of excessive contrast and disharmony with characteristic landscape.

Inundate: To cover with impounded waters or floodwaters.

Inundation map: A map delineating the area that would be flooded by a particular flood event. It includes the ground surfaces downstream of a dam showing the probable encroachment by water released because of failure of the dam or from abnormal flood flows released through a dam's spillway and/or other appurtenant works.

Invert: The lowest point of an underground excavation or the lowest point of the interior of a circular conduit, pipe, or tunnel. The lowest portion of the inside of any horizontal pipe.

Invertebrates: All animals without a vertebral column, e.g. spiders, crabs, or worms.

Ion: An atom or molecule that has lost or gained one or more electrons.

Iowa vane: A flow deflector that will divert water in a way calculated to attract fish.

Irretrievable: Commitments that are lost for a period of time.

Irreversible: Commitments that cannot be reversed, except perhaps in the extreme long term.

Irrigable acreage for service (irrigable area for service): The acreage classified as irrigable for which project works have been constructed and project water is available. This acreage may change from year to year, generally increasing, as project works are completed and service is made available to additional acreage. Upon completion of the project, the irrigable acreage for service will equal the irrigable land as presented in the repayment contract or most recent project authorization. See irrigated acreage. Lands that have not been classified but which are furnished water under Special or Warren Act contracts and lease or water rental agreements are shown as a part of the irrigable area for service.

Irrigable area not for service: Part of the total irrigable area for which service eventually will be available but for which project facilities have not yet been constructed.

Irrigable land: Arable land under a specific plan for which a water supply is or can be made available and which is provided with or planned to be provided with irrigation, drainage, flood protection, and other facilities as necessary for sustained irrigation.

Irrigated acreage: Irrigable acreage actually irrigated in any one year. It includes irrigated cropland harvested, irrigated pasture, cropland planted but not harvested, and acreage in irrigation rotation used for soil-building crops.

Irrigation: Act of supplying dry land with water in order to grow crops or other plants. Application of water to lands for agricultural purposes.

Irrigation check: Small dike or dam used in the furrow alongside an irrigation border to make the water spread evenly across the border.

Irrigation district: A cooperative, self-governing public corporation set up as a subdivision of the State government, with definite geographic boundaries, organized and having taxing power to obtain and distribute water for irrigation of lands within the district; created under the authority of a State legislature with the consent of a designated fraction of the landowners or citizens.

Irrigation efficiency: The ratio of the average depth of irrigation water that is beneficially used to the average depth of irrigation water applied, expressed as a percent. Beneficial uses include satisfying the soil water deficit and any leaching requirement to remove salts from the root zone.

Irrigation requirement: Quantity of water, exclusive of effective precipitation, that is required for crop production. See crop irrigation requirement.

Isotropic mass: A mass having the same property (or properties) in all directions.

Jeopardy opinion: U.S. Fish and Wildlife Service (FWS) or National Marine Fisheries Service (NMFS) opinion that an action is likely to jeopardize the continued existence of a listed species or result in the destruction or adverse modification of critical habitat. The opinion includes reasonable and prudent alternatives, if any. See no jeopardy opinion.

Jet-flow gate: A gate consisting of a wheel-mounted leaf moved vertically by a motor-driven screw hoist. High pressure gate resembling a ring follower gate in general configuration, but designed for regulating flow with minimal cavitation damage.

Jetting: A method of compacting soil using a hose or other device, with a high velocity stream of water, worked down through the depth of soil placed. Drilling with high pressure water or air jets.

Jetting pump: A water pump that develops very high discharge pressure.

Jetty: Pier or other structure built out into a body of water to influence the current or tide, or to protect a harbor or shoreline. A long fill or structure extending into water from the shore, that serves to change the direction or velocity of water flow.

Job hazard analysis (JHA): A study of a job or activity to identify hazards or potential accidents associated with each step or task, and develop solutions that will eliminate, nullify, or prevent such hazards or accidents.

Jogging: The frequent starting and stopping of an electric motor.

Joint. See expansion joint, cold joint, construction joint, contraction joint, and control joint.

Joint information center: A single location where public information officials gather to collaborate on, and coordinate the release of, emergency public information.

Joint use capacity: The reservoir capacity assigned to flood control purposes during certain periods of the year and to conservation purposes during other periods of the year.

Jurisdiction: Boundary of authorization for a State, county, and/or city emergency management agency.

Juvenile: Young fish older than 1 year but not capable of reproduction.

Juvenile salmon: All early lifestages of downstream migrating salmon (fry through smolt).

Kilovolt (kV): One thousand volts.

Kilovolt-ampere (kVA): 1000 volt-amperes and approximately 89/100 of a kilowatt.

Kilowatt (kW): An electrical unit of work or power. Unit of electric power (capacity) equal to 1000 watts, or about 1.34 horsepower, and 1.18 KVA.

Kilowatt-hour (kWh): Basic unit of electric energy equal to an average of one kilowatt of power applied over one hour. A unit of energy equivalent to one thousand watthours.

Lacustrine habitat: Lake or reservoir wetland habitat.

Lagging: In tunneling, planking placed against the dirt or rock walls and ceiling, outside the ribs.

Lagomorphs: The order of mammals including rabbits, hares, and pikas.

Laminar flow: The flow field can be characterized by layers of fluid, one layer not mixing with adjacent ones. The flow is laminar or turbulent depending on the value of the Reynolds number, which is a dimensionless ratio of the inertial forces to the viscous forces. In laminar flow, viscous forces are dominant and the Reynolds number is relatively small. In turbulent flow, the inertial forces are very much greater than the viscous forces and the Reynolds number is large. Laminar flow occurs very infrequently in open channel flow.

Land classification: Reclamation's systematic placing of lands into classes based on their suitability for sustained irrigated farming. Land classes are defined by productivity, with Class 1 being the most productive. For other classes, the equivalent acreage to Class 1 for the same productivity is defined (Class 1 equivalency). For example, (the productivity of) X acres of Class 2 land is equal to (the productivity of) 1 acre of Class 1 land.

Landfill: An open area where trash is buried. Facility in which solid waste from municipal and/or industrial sources is disposed; sanitary landfills are those that are operated in accordance with environmental protection standards.

Landholder: A landholder is an individual or legal entity owning or leasing land subject to the acreage limitation provisions.

Land ownership: Land held in title.

Land retirement: Permanent removal of land from agricultural production.

Landslide: The failure of a sloped bank of soil or rock in which the movement of the mass takes place along a surface of sliding.

Larval fish: An immature stage that develops from the fertilized egg before assuming the characteristics of the adult.

Lateral: A channel that conveys water from a canal to a farm, municipality, etc.

Leach: To remove components from the soil by the action of water trickling through.

Leaching: Removal of soluble material from soil or other permeable material by the passage of water through it. The removal of soluble soil material and colloids by percolating water.

Leaching field (leaching cesspool): A lined or partially lined underground pit into which raw household water (sewage) is discharged and from which the liquid seeps into the surrounding soil.

Leaching requirement: Quantity of irrigation water required for transporting salts through the soil profile to maintain a favorable salt balance in the root zone for plant development.

Lead time: Time available for Decision Makers to determine if conditions at a dam and reservoir, or in the basin, warrant declaration of a specific emergency classification level.

Leakage: Free flow loss of water through a hole or crack.

Lease of power privilege: Contractual right given to a non-Federal entity to utilize, consistent with project purposes, water power head and storage from Reclamation projects for electric power generation.

Left abutment: See abutment.

Left or right designation: In the context of dams and river channels, the designation of left or right is made with the observer looking in the downstream direction.

Leg: A side post in tunnel timbering.

Length of dam. See crest length.

Lentic: Standing waters, such as lakes, ponds, and marshes.

Levee: A natural or man-made barrier that helps keep rivers from overflowing their banks. See dike.

Level: To make level or to cause to conform to a specified grade. Any instrument that can be used to indicate a horizontal line or plane.

Level of protection (LOP): As used in the evaluation process, it is the level of loading selected to which corrective actions will be designed to prevent dam failure.

Life cycle: Various stages an animal passes through from egg fertilization to death.

Lift: A step or bench in a multiple layer excavation or fill.

Lift line: Horizontal construction joint created when new concrete is placed on previously placed concrete.

Limnology: Scientific study of the physical characteristics and biology of lakes, ponds, and streams.

Limnological conditions: Conditions on freshwater lakes.

Lineament: A rectilinear topographic feature.

Lining: Any protective material used to line the interior surface of a conduit, pipe, or tunnel. With reference to a canal, tunnel or shaft, a coating of asphaltic concrete, concrete, reinforced concrete, or shotcrete to provide watertightness, to prevent erosion, or to reduce friction. Protective covering over the perimeter of a conduit, reservoir, or channel to prevent seepage losses, to withstand pressure, or to resist erosion.

Liquefaction:: When a solid form is turned into a liquid form. During an earthquake, low density materials act like water and lose their supporting strength. A condition whereby soil undergoes continued deformation at a constant low residual stress or with low residual resistance, due to the buildup and maintenance of high pore water pressures, which reduces the effective confining pressure to a very low value. Pore pressure buildup leading to liquefaction may be due either to static or cyclic stress applications, and the possibility of its occurrence will depend on the void ratio or relative density of a cohesionless soil and the confining pressure.

Liquid limit: The water content corresponding to the arbitrary limit between the liquid and plastic states of consistency of a soil; the water content at which a pat of soil, cut by a groove of standard dimensions, will flow together for a distance of 1/2 inches under the impact of 25 blows in a standard liquid limit apparatus. Minimum moisture content which will cause soil to flow if jarred slightly.

Littoral: Pertaining to the shore.

Littoral zone: The zone between the high and low water marks.

Live capacity (live storage): That part of the total reservoir capacity which can be withdrawn by gravity. This capacity is equal to the total capacity less the dead capacity.

Load: Amount of electrical capacity or energy delivered or required at a given point. The power output of an engine or powerplant under given circumstances.

Load factor: The ratio of an average load to the maximum load. Average load carried by an engine, machine, or plant, expressed as a percentage of its maximum capacity.

Loam: A soft, easily worked soil containing sand, silt, and clay.

Local capabilities assessment: Evaluation and report performed by members of the Early Warning System design team and/or community planners to assess the warning and evacuation capabilities of local jurisdictions located downstream from Reclamation dams.

Local emergency operations plan (LEOP): A general planning document, required by law, that describes the responsibilities and actions to be performed in the event of an emergency and/or disaster (also refer to as emergency operations plans).

Local emergency planning committee (LEPC): A committee made up of local officials, citizens, and industry representatives charged with development and maintenance of emergency response plans for the local emergency planning district as per SARA Title III requirements. Planning procedures include hazardous materials inventories, plans, hazardous material response training, and assessment of local response capabilities.

Local officials/authorities: The personnel authorized by election or job title to carry out the operation, planning, mitigation, preparedness, response, and recovery functions of the emergency management programs at local levels.

Local runoff: Water running off a local area, such as rainfall draining into a nearby creek.

Lockout: The placement of a lockout device on an energy isolating device, in accordance with an estabilshed procedure, ensuring that the energy isolating device and the equipment being controlled cannot be operated until the lockout device is removed. Clearance procedure in which physical locks replace Safety Tags and prevent operating switches, controls, etc. See tagout and clearance.

Lockout device: A device that utilizes a positive means such as a lock, either key or combination type, to hold an energy isolating device in the safe position and prevent the energizing of a machine or equipment. Included are blank flanges and bolted slip blinds.

Loess: Wind-deposited silt. A uniform aeolian deposit of silty material having an open structure and relatively high cohesion due to cementation of clay or calcareous material at grain contacts. A characteristic of loess deposits is that they can stand with nearly vertical slopes.

Loessial: Medium-textured materials (usually silt or very fine sand) transported and deposited by wind action. These materials may be deposited in depths ranging from less than 1 foot to well over 100 feet.

Log boom: A floating structure used to protect the face of a dam by deflecting floating material and waves away from the dam. A device used to prevent floating debris from obstructing spillways and intakes. A chain of logs, drums, or pontoons secured end to end and floating on the surface of a reservoir so as to divert floating debris, trash, and logs.

Logbook (operating log): A dated, written record of performed operation and maintenance items or observations pertinent to a structure.

Long-throated flumes: Long-throated flumes control discharge rate in a throat that is long enough to cause nearly parallel flow lines in the region of flow control. Parallel flow allows these flumes to be accurately rated by analysis using fluid flow concepts. Long-throated flumes can have nearly any desired cross-sectional shape and can be custom fitted into most canal-site geometries. The Ramp flume, also considered a version of broad-crested weir, is an example of this kind of flume.

Loose yards: Measurement of soil or rock after it has been loosened by digging or blasting.

Losing stream: A stream or reach that contributes water to a zone of saturation.

Lotic: Flowing water, such as rivers and streams.

Low dam: A dam up to 100 feet high. See medium-height dam and high dam.

Low density material: A material having a low weight per unit volume either as it occurs in its natural state or after compacting it, such as loess with a density of 80 pounds per cubic foot or less.

Low hazard: A downstream hazard classification for dams in which no lives are in jeopardy and minimal economic loss (undeveloped agriculture, occasional uninhabited structures, or minimal outstanding natural resources) would occur as a result of failure of the dam. This classification also applies to structures other than dams.

Lysimeter: An isolated block of soil, usually undisturbed and in situ, for measuring the quantity, quality, or rate of water movement through or from the soil.

Macroclimate: The climate representative of relatively large area.

Macrohabitat: An extensive habitat presenting considerable variation of the environment, containing a variety of ecological niches and supporting a large number and variety of complex flora and fauna.

Macrophytes: A plant large enough to be seen by the naked eye.

Magnitude: A rating of a given earthquake, independent of the place of observation. It is calculated from measurements on seismographs and it is expressed in ordinary numbers and decimals based on a logarithmic scale. A measure of the strength of an earthquake, or the strain energy released by it, as determined by seismographic observations. See Richter scale.

Main: The principal pipe artery to which branches may be connected.

Main channel: The deepest or central part of the bed of a stream, containing the main current.

Main channel pool: Reach of a stream or river with a low bed elevation, relative to rapids or riffles.

Mainstream (mainstem): The main course of a stream where the current is the strongest.

Maintenance: All routine and extraordinary work necessary to keep the facilities in good repair and reliable working order to fulfill the intended designed project purposes. Maintaining structures and equipment in intended operating condition, equipment repair, and minor structure repair.

Maintenance management system: Any organized system used to ensure that all operations and maintenance activities (e.g., maintenance, inspection, operational testing) at a facility is accomplished and documented.

Major facility: A term used by Reclamation to describe those facilities for which an examination is conducted every third year, alternately conducted by the Denver and respective regional office. Major facilities include storage dams and reservoirs, diversion dams with significant storage or where major equipment and operation are complex, large pumping plants and powerplants, large canal systems, large complex closed conduit systems, and Group A bridges.

Manifold (Header): A large pipe to which a series of smaller pipes are connected.

Manning's roughness coefficient (n): A coefficient used to describe the relative roughness of a channel and overbank areas; used in hydraulic computations.

Map: Usually a two-dimensional representation of all or part of the Earth's surface showing selected natural or manmade features or data, preferably constructed on a definite projection with a specified scale.

Marginal land: Land which, in its natural state, is not well suited for a particular purpose, such as raising crops.

Masonry dam: Any dam constructed mainly of stone, brick or concrete blocks jointed with mortar. A dam having only a masonry facing should not be referred to as a masonry dam. Masonry dams differ from rockfill dams in that the stone is hand-placed with mortar resulting in the entire dam being impermeable.

Mass care center: A facility for providing emergency lodging and care for people made temporarily homeless by an emergency or disaster. Essential basic services (feeding, family reunification, etc.) are provided.

Mass Concrete: Any large volume of concrete cast-in-place, generally as a monolithic structure. Dimensions of the structure are of such magnitude that measures must be taken to cope with the generation of heat and the resulting volume changes and cracking.

Massive head buttress dam: A buttress dam in which the buttress is greatly enlarged on the upstream side to span the gap between buttresses. See solid head buttress dam.

Mastic: A soft sealing material.

Maximum Contaminant Level (MCL): The highest level of a contaminant that is allowed in drinking water. MCL's are set as close to the Maximum Contaminant Level Goal as feasible using the best available treatment technology.

Maximum Contaminant Level Goal (MCLG): The level of a contaminant in drinking water below which there is no known or expected risk to health.

Maximum controllable water surface: The highest reservoir water surface elevation at which gravity flows from the reservoir can be completely shut off.

Maximum credible earthquake (MCE): The largest hypothetical earthquake that may be reasonably expected to occur along a given fault or other seismic source could produce under the current tectonic setting. It is a believable event which can be supported by all known geologic and seismologic data. A hypothetical earthquake is deterministic if its fault or source area is spatially definable and can be located a particular distance from the dam under consideration. A hypothetical earthquake is probabilistic if it is considered to be a random event, and its epicentral distance is determined mathematically by relationships of recurrence and magnitude for some given area. The MCE can be associated with specific surface geologic structures and can also be associated with random or floating earthquakes (movements that occur at depths that do not cause surface displacements). The seismic evaluation criteria determines which faults or seismic sources are assigned an MCE. The most severe earthquake that can be expected to occur at a given site on the basis of geologic and seismological evidence. The severest earthquake that is believed to be possible at the site on the basis of geologic and seismological evidence. It is determined by regional and local studies that include a complete review of all historic earthquake data of events sufficiently nearby to influence the project, all faults in the area, and attenuations from causative faults to the site. The earthquake associated with specific seismotectonic structures, source areas, or provinces that would cause the most servere vibratory ground motion or foundation dislocation capable of being produced at the site under the currently known tectonic framework.

Maximum design earthquake (MDE): The earthquake selected for design or evaluation of the structure. This earthquake would generate the most critical ground motions for evaluation of the seismic performance of the structure among those loadings to which the structure might be exposed. For example, if a site were assigned MCEs from two separate sources, the MCE which would be expected to generate the most severe ground motions would be the maximum design earthquake. The response of the structure to specific ground motion parameters (frequency, duration, etc.) needs to be considered in specifying this event. In certain cases, more than one maximum design earthquake may be specified to reflect the differing response of various components of the structure to earthquake loading.

Maximum flood control level: The highest elevation of the flood control storage.

Maximum water surface (maximum pool): The highest acceptable water surface elevation with all factors affecting the safety of the structure considered. It is the highest water surface elevation resulting from a computed routing of the inflow design flood through the reservoir under established operating criteria. This surface elevation is also the top of the surcharge capacity.

Maximum wave: The highest wave in a wave group.

Meander: Big bend and loops in a river channel as the river snakes through a flat land area.

Mean number: The average number.

Mean sea level (msl): The elevation of the ocean halfway between high and low tide.

Median: Middle value in a distribution, above and below which lie an equal number of values.

Medium-height dam: A dam between 100 and 300 feet high. See low dam and high dam.

Medium-size water system: A water system that serves greater than 3,300 and less than or equal to 50,000 persons.

Medium-thick arch dam: An arch dam with a base thickness to structural height ratio between 0.2 and 0.3 (previously defined as between 0.3 and 0.5).

Megawatt (MW): One million watts of electrical power (capacity).

Megawatt-hour (MWh): One million watt- hours of electrical energy.

Membrane (diaphragm): A membrane or sheet or thin zone or facing, made of a flexible impervious material such as asphaltic concrete, plastic concrete, steel, wood, copper, plastic, etc. A cutoff wall or core wall, if thin and flexible, is sometimes referred to as a "diaphragm wall."

Memorandum of understanding (MOU): A formal document that states the intentions and/or responsibilities of the signatory parties. A memorandum of understanding does not provide the authority to transfer any funding from Reclcamtion to another party, but may cover Reclamation services reimbursed by others. Also letter of agreement.

Mesh: In wire screen, the number of openings per lineal inch.

Mesotrophic: Reservoirs and lakes which contain moderate quantities of nutrients and are moderately productive in terms of aquatic animal and plant life. See oligotrophic.

Metalimnion: Middle layer of a lake or reservoir with a rapid temperature decrease with depth. See stratification and thermocline.

Metamorphic: Rock compressed or changed by pressure, heat, or water.

Microclimate: The climate of a small area, particularly that of the living space of a certain species, group or community.

Microhabitat: A small, specialized, and effectively isolated location. See macrohabitat.

Microsystem irrigation: Method of precisely applying irrigation water to the immediate root zone of the target plant at very low rates.

Migratory: Moving from one area to another on a seasonal basis.

Milestone: A measurable action, state, or goal which marks a point of achievement on the way to solving the problem.

Military crest: A ridge that interrupts the view between a valley and a hilltop.

Mill: Monetary cost and billing unit used by utilities; equal to 1/1000 of U.S. dollar (or 1/10 of one cent).

Milligram per liter (mg/l): Equivalent to 1 part per million.

Million acre-feet (maf): The volume of water that would cover 1 million acres to a depth of 1 foot.

Millisecond delay (short period delay): A type of delay cap with a definite but extremely short interval between initiation, or passing of current, and explosion.

Mine tailings dam: An industrial waste dam in which the waste materials come from mining operations or mineral processing.

Minimum flow: Negotiated lowest flow in a regulated stream that will sustain an aquatic population of agreed-upon levels. Flow may vary seasonally. Lowest flow in a specified period of time.

Minimum operating level: The lowest level to which the reservoir is drawn down under normal operating conditions.

Mining: Usually removal of soil or rock having value because of its chemical composition.

Mitigation (measures): Action taken to avoid, reduce the severity of, or eliminate an adverse impact. Mitigation can include one or more of the following:

Avoiding impacts.

Minimizing impacts by limiting the degree or magnitude of an action.

Rectifying impacts by restoration, rehabilitation, or repair of the affected environment.

Reducing or eliminating impacts over time.

Compensating for the impact by replacing or providing substitute resources or environments to offset the loss.

Mixed face: In tunneling, digging in dirt and rock in the same heading at the same time.

Moderate frequency flood: A flood of lesser magnitude than the IDF, used for the service spillway design when supplemented by a separate auxiliary spillway.

Modification Decision Analysis (MDA): The process of determining with confidence whether dam safety deficiencies exist for maximum loading conditions. The MDA process is a technical, state-of-the-art evaluation whose complexity can range from the simplified SEED Analysis Report evaluations using available data, to advanced engineering and geologic analyses conducted using material properties and other data obtained from detailed field and laboratory investigations. The MDA evaluates suspected safety of dam deficiencies, identifies the critical types of loading conditions that may cause dam failure, (e.g., earthquake, flood, and static reservoir loading), and eliminates or verifies suspected deficiencies which are capable of causing failure of the dam.

Modified homogeneous earthfill dam: A homogeneous earthfill dam that uses pervious material specially placed in the embankment to control seepage. See embankment dam.

Modified Mercalli scale: An earthquake intensity scale which has twelve divisions ranging from I (not felt by people) to XII (nearly total damage). For more information on the modified Mercalli scale, visit the U.S. Geological Survey National Earthquake Information Center.

Mohr circle: A graphical representation of the stresses acting on the various planes at a given point.

Moisture content (water content) The ratio of the weight of water in a soil sample to the weight of the dry soil, expressed as a percentage. See optimum moisture content.

Monocoque gate: A thin-shell radial gate in which the usual skin plate and cross-beam framework are replaced by a hollow shell having an approximate elliptical cross section. This type of gate deviates considerably from the conventional beam and skin-plate design in that the shell itself is relied upon to withstand the beam action.

Monomictic lakes: Warm- water lakes which turn over annually, usually in winter, and where the temperature never falls below 4 degrees C.

Morning glory spillway: A circular or glory hole form of a drop inlet spillway. Usually free standing in the reservoir and so called because of its resemblance to the morning glory flower.

MS connectors: Surface delays for use when shooting with detonating cord.

Muck: Mud rich in humus. Stone, dirt, debris, or useless material; or an organic soil of very soft consistency. Finely blasted rock, particularly from underground.

Mud: Generally, any soil containing enough water to make it soft. A mixture of soil and water in a fluid or weakly solid state.

Mudflat: A mud-covered, gently sloping tract of land alternately covered and left bare by water. The muddy, nearly level bed of a dry lake.

Mulch: Material spread on the ground to reduce soil erosion and evaporation of water.

Multiple arch dam: A buttress dam, the upstream part of which comprises a series of arches.

Multiple-purpose reservoir (multipurpose reservoir): A reservoir planned to operate for more than one purpose.

Multipurpose dam: A dam constructed for two or more purposes (e.g. storage, flood control, navigation, power generation, recreation, or fish and wildlife enhancement.)

Multipurpose project: A project designed for irrigation, power, flood control, municipal and industrial, recreation, and fish and wildlife benefits, in any combinations of two or more (contrasted to single-purpose projects serving only one need).

Nameplate: Power generation capacity of a generator that can be guaranteed under continuous operation.

National disaster medical system: A system designed to deal with extensive medical care needs in very large disasters or emergencies. The system is a cooperative effort of the Department of Health and Human Services, Federal Emergency Management Agency, Department of Defense, State and local governments, and the private sector.

National Environmental Policy Act (NEPA): An act requiring analysis, public comment, and reporting for environmental impacts of Federal actions.

National Institute for Occupational Safety and Health (NIOSH): NIOSH is the Federal agency responsible for conducting research and making recommendations for the prevention of work-related disease and injury.

National Oceanic and Atmospheric Administration (NOAA): The National Oceanic and Atmospheric Administration's mission is to describe and predict changes in the Earth's environment, and conserve and wisely manage the Nation's coastal and marine resources.

National Park Service (NPS): The National Park Service preserves unimpaired the natural and cultural resources and values of the national park system for the enjoyment, education, and inspiration of this and future generations. NPS is an agency within the U.S. Department of the Interior.

National Pollutant Discharge Elimination System (NPDES): A permitting program under section 402 of the Clean Water Act required for all point sources discharging pollutants into waters of the United States. The purpose of the NPDES program is to protect human health and the environment.

National Register of Historic Places: A federally maintained register of districts, sites, buildings, structures, architecture, archeology, and culture. Visit the National Register of Historic Places website.

National Response Center (NRC): Located at the U.S. Coast Guard Headquarters in Washington, D.C., the NRC is a 24-hour national communications center and the single point of contact for pollution - incident reporting. It is also the National Response Team's communications center. Immediate reporting is required for reportable quantities of petroleum or hazardous substances 40 CFR 300.125(c).

National Response Team (NRT): NRT consists of representatives from several Federal agencies including the Department of the Interior (DOI) and the Bureau of Reclamation. NRT is primarily a policy board which provides program direction, planning and preparedness guidance, and review of regional response activites. NRT should be activated as an emergency response team when an oil or hazardous substance release exceeds the response capability of the region(s), transects regional boundaries; or involves a significant threat to public health or welfare or the environment, substantial amounts of property, or substantial threats to natural resources.

National Warning System (NAWAS): A dedicated, commercially leased, nationwide voice telephone warning system operated on a 24-hour basis, with a National Warning Center and an Alternate National Warning Center staffed by attack warning officers. Special purpose telephone circuits connect the National and Alternate National Warning Centers (NAWAS) to the following: Federal Emergency Management Agency headquarters and regional offices; selected Federal departments and agencies; military installations; and State, county, and city warning points. NAWAS is the only national system designed and maintained to warn the public of a nuclear attack or a natural or manmade disaster.

National Water Quality Assessment Program: Describes current water quality conditions for a large part of the United States freshwater stream, rivers, and aquifers. Describes how water quality is changing over time. Improves understanding of the primary natural and human factors that affect water quality conditions.

National Weather Service (NWS): The National Weather Service provides weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy.

Natural floodway: The channel of a water course and those portions of the adjoining flood plain which are reasonably required to carry a selected probability flood.

Natural frequency (f): The natural frequencies of a structure are the frequencies of free vibration. Free vibration is vibration that occurs in the absence of forced vibration. In a structure undergoing vibration, a mode of vibration is a characteristic pattern (shape) assumed by the structure in which the motion of every particle is simple harmonic motion with the same frequency. The fundamental mode of vibration of a structure is the mode having the lowest natural frequency.

Natural period of vibration (T): The period of vibration of a a structure is the time required for one cycle of the simple harmonic motion in one of these characteristic patterns (shapes). T = 1/f.

Naturalness: Area which generally appears to have been affected primarily by the forces of nature, with the imprint of man's work substantially unnoticeable.

Navigable waters: Waters of the United States including:

(a) All waters that are currently used, were used in the past, or may be susceptible to use in interstate or foreign commerence, including all waters that are subject to the ebb and flow of the tide.

(b) Interstate waters, including interstate wetlands.

(c) All other waters such as intrastate lakes, rivers, streams (including intermittent streams), mudflats, sandflats and wetlands, the use, degradation, or destruction of which would affect or could affect interstate or foreign commerce, including waters used or which could be used for industries in interstate commerce.

(d) All impoundments of waters otherwise defined as navigable waters.

(e) Tributaries of waters identified in (a) through (d).

(f) Wetlands adjacent to waters identified in (a) through (d).

Neatlines (of structure): A line which defines the limits of work, such as an excavation, cut stone, etc. Also, the true face line of a building regardless of the projections of the stones; a line back of, or inside of, incidental projections. Any material removed beyond the neat line is overbreak.

Needle valve: Any of a family of valves which regulate flow through the use of a needle moving into and out of an orifice. Types include the Ensign Valve, Balanced Needle Valve, Internal Differential Needle Valve, and Interior Differential Needle Valve.

Negative pressure: Pressure within a pipe that is less than atmospheric pressure.

Neoprene: A synthetic rubber with superior resistance to oils; often used as gasket and washer material.

Neotropic migrant (neotropical): A bird that migrates to tropical regions during the winter.

Net cut: The cut required, less the fill required, at a particular station or part of a road.

Net economic benefits: Economic benefits less economic costs.

Net fill: The fill required, less the cut required, at a particular station or part of a road.

Neutron probe: An instrument used to estimate soil moisture. Relates the rate of attenuation in pulsed neutron emissions to soil water content.

New York rod: A leveling rod marked with narrow lines, ruler-fashion.

Nipple: A short piece of pipe with male threads on each end.

No jeopardy opinion: U.S. Fish and Wildlife Service (FWS) or National Marine Fisheries Service (NMFS) opinion that an action is not likely to jeopardize the continued existence of listed species or result in the destruction or adverse modification of critical habitat. See jeopardy opinion.

Nominal dollars: Dollar value in the indicated year, not adjusted for inflation.

Nominal size: The approximate dimension(s) of standard materials.

Nonconsumptive water uses: Water uses that do not substantially deplete water supplies, including swimming, boating, waterskiing, fishing, maintenance of stream related fish and wildlife habitat, and hydropower generation.

Nonfirm power: Power that is not available continuously and may be interruptible. This power may be marketed on a short-term basis.

Nonpoint source: A contributing factor to water pollution that cannot be traced to a specific spot. Man-made or man-induced alteration of the chemical, physical, biological, or radiological integrity of water, originating from any source other than a point source. Pollution which comes from diffuse sources such as urban and agricultural runoff. Major non point sources of pollution include excess farm and lawn nutrients that move through the soil into the ground water or enter local water directly through runoff during heavy rains; uncontrolled storm water runoff from construction sites; forestry operations; animal wastes; and even pollutants released directly into the atmosphere.

Non-potable: Water that may contain objectionable pollution, contamination, minerals, or infective agents and is considered unsafe and/or unpalatable for drinking.

Non-reimbursable: Cost of constructing, operating, or maintaining a Reclamation project that is borne by the Federal taxpayer and is not reimbursed by any other individual, entity, or organization.

Nonterritorial communities: Networks of associations around shared goals, values, and norms, such as the agricultural, environmental, or recreational community; also known as "special interest communities."

Non-uniform flow: The velocity varies with position.

Non-utility generators (NUG's): Facilities for generating electricity that are not owned exclusively by an electric utility and which operate connected to an electric utility system.

Normal depth: The depth of flow that would exist for a steady-uniform flow condition.

Normal loading conditions: Loading conditions that occur or are anticipated to occur with some degree of regularity or frequency, as constrasted to unusual loading conditions such as an MCE or PMF.

Normal water surface: The highest elevation that water is normally stored, or that elevation which the reservoir should be operated for conservation purposes. Usually the elevation at the top of the active conservation capacity. The maximum elevation to which the reservoir may rise under normal operating conditions exclusive of flood control capacity.

North American Electric Reliability Council (NERC): Principal organization for coordinating and promoting reliability for North America's electric utilities. NERC was formed in 1968 in the aftermath of the November 9, 1965 northeast blackout.

Nutrients: Animal, vegetable, or mineral substance which sustains individual organisms and ecosystems. Any substance that is assimilated by organisms and promotes growth.

Oakum: Loosely woven hemp rope that has been treated with oil or other waterproofing agent; it is used to caulk joints in a bell and spigot pipe and fittings.

Obligate riparian species: A species that depends completely upon habitat along a body of water.

Observation well: A hole used to observe the ground-water sufrace at atmospheric pressure within soil or rock.

Occupational Safety and Health Act of 1970 (OSHA): The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA) is a law designed to protect the health and safety of industrial workers and also the operators of water supply systems and treatment plants. OSHA also refers to the federal and state agencies which administer the OSHA regulations.

Off-peak energy: Electric energy supplied during periods of relatively low system demand.

Ogee crest: The shape of the concrete spillway crest that represents the lower profile of the undernappe of a jet of water flowing over a sharp-crested weir at a design depth.

Ohm: The unit of electrical resistance to current flow. The value of resistance through which a potential difference of one volt will maintain a current of one ampere.

Oil: Defined as oil of any kind or in any form including, but not limited to, petroleum, fuel oil, sludge, oil refuse, and oil mixed with wastes.

Oil spill contingency fund: A revolving fund for spill control efforts has been authorized in cases where the Federal government has taken over containment and cleanup operations. The fund is administered by the U.S. Coast Guard. Once the responsible party is determined, they are required to reimburse the fund for the oil removal costs.

Oligotrophic: Reservoirs and lakes which are nutrient poor and contain little aquatic plant or animal life. See mesotrophic.

Omnivore: Animal that eats both vegetable and animal substances.

On-district storage: Small water storage facilities located within the boundaries of an irrigation entity, including reregulating reservoirs, holding ponds, or other new storage methods that allow for efficient water use.

On-farm: Activities (especially growing crops and applying irrigation water) that occur within the legal boundaries of private property.

On-farm irrigation efficiency: The ratio of the volume of water used for consumptive use and leaching requirements in cropped areas to the volume of water delivered to a farm (applied water).

On-peak energy: Electric energy supplied during periods of relatively high system demand.

On-scene coordinator (OSC): The Federal official predesignated by the EPA or Coast Guard to coordinate and direct Federal removal efforts at the scene of an oil or hazardous substance discharge (as prescribed in the National Oil and Hazardous Substances Pollution Contingency Plan and as published in 40 CFR Part 300). In an emergency, the OSC may authorize spending up to $2,000,000 for a removal action.

Open-cut: A method of excavation in which the working area is kept open to the sky. Used to distinguish from cut-and-cover and underground work.

Open-work materials: Poorly-graded (uniform or gap-graded gradation) gravels, cobbles and boulders with few fines in the matrix, resulting in a deposit containing a large amount of interconnected void space through which seepage water (and soil particles) can easily move.

Operating basis earthquake (OBE): The earthquake that the structure must safely withstand with no damage. All systems and components necessary to the uninterrupted functioning of the project are designed to remain operable during the ground motions associated with the OBE. This includes the dam, appurtenant structures, electrical and mechanical equipment, relays, spillway gates, and valves. For most usage in Reclamation, the OBE is specified to have a 90% probability of nonoccurrence in a 25-year-exposure period. This is equivalent to a recurrence interval of 237 years. Economic considerations for specific projects may lead to consideration of other values.

Operating log. See logbook.

Operational losses: Losses of water resulting from evaporation, seepage, and spills.

Operational waste: Water that is lost or otherwise discarded from an irrigation system after having been diverted into it as part of normal operations.

Operations and maintenance (O&M): Operation, maintenance, repairs, replacements, testing, and exercising of any or all portions of an Early Warning System for the life of the system.

Optimum moisture content, or Optimum water content: The one water content (percent of dry weight of the total material) of a given soil and a given compactive effort that will result in a maximum dry density of the soil.

Option value: Value associated with people who know they can visit an area in the future if they so desire. Also a reversible decision or an option to develop at some time in the future have option value.

Ore: Rock or earth containing workable quantities of a mineral or minerals of commercial value.

Original ground (surface): The surface of the earth as it exists in an unaltered state (i.e. prior to any earthwork). See existing ground.

O-ring: A rubber seal used around stems of some valves to prevent water from leaking past.

Organic: Any chemical containing the element carbon.

Outage: Period during which a generating unit, transmission line, or other facility is out of service.

Outflow: The amount of water passing a given point downstream of a structure, expressed in acre-feet per day or cubic feet per second. Water flowing out of a body of water.

Outlet: An opening through which water can be freely discharged from a reservoir to the river for a particular purpose.

Outlet capacity: The amount of water that can be safely released through the outlet works.

Outlet channel (exit channel): Channel downstream from terminal structure that conveys releases back to the "natural" stream or river. Channel can be excavated in rock or soil, with or without riprap, soil cement or other types of erosion protection.

Outlet gate: A gate controlling the flow of water through a reservoir outlet.

Outlet Works: A combination of structures and equipment required for the safe operation and control of water released from a reservoir to serve various purposes, i.e., regulate stream flow and quality; release floodwater; and provide irrigation, municipal, and/or industrial water. Included in the outlet works are the intake structure, conduit, control house-gates, regulating gate or valve, gate chamber, and stilling basin. A series of components located in a dam through which normal releases from the reservoir are made. A device to provide controlled releases from a reservoir. A pipe that lets water out of a reservoir, mainly to supply downstream demands.

Outlet works tower: A tower within a reservoir that contains the mechanisms to open the entrance to the outlet works.

Overall safety of dams classification: One of the following classifications is assigned to a dam following an onsite examination and subsequent analyses using available data and state-of-the-art knowledge:

Satisfactory. - No existing or potential dam safety deficiencies are recognized. Safe performance is expected under all anticipated loading conditions, including such events as the maximum credible earthquake (MCE) and the probable maximum flood (PMF).

Fair. - No existing dam safety deficiencies are recognized for normal loading conditions. Infrequent hydrologic and/or seismic events would probably result in a dam safety deficiency.

Conditionally Poor. - A potential dam safety deficiency is recognized for unusual loading conditions that may realistically occur during the expected life of the structure. Conditionally Poor may also be used when uncertainties exist as to critical analysis parameters that identify a potential dam safety deficiency; further investigations and studies are necessary.

Poor. - A potential dam safety deficiency is clearly recognized for normal loading conditions. Immediate actions to resolve the deficiency are recommended; reservoir restrictions may be necessary until the problem is resolved.

Unsatisfactory. - A dam safety deficiency exists for normal loading conditions. Immediate remedial action is required for problem resolution.

Overamping: Exceeding the rated capacity of a system.

Overbreak: Moving or loosening of rock as a result of a blast, beyond the intended line of cut.

Overburden: Soil or rock lying on top of a pay formation.

Overdraft: The pumping of water from a ground water basin or aquifer in excess of the supply flowing into the basin. This pumping results in a depletion or "mining" of the ground water in the basin.

Overexcavation: Excavation beyond specified or directed excavation. Removing unsuitable foundation material.

Overhang: Projecting parts of a face or bank.

Overflow dam: A dam designed to be overtopped.

Overflow spillway (ogee spillway): A spillway that has a control weir that is ogee-shaped (S-shaped) in profile.

Overhaul: Movement of (earth) material far enough so that payment, in addition to excavation pay, is made for haulage. The distance in excess of that given as the stated haul distance to haul excavated material.

Overtopping: Flow of water over the top of a dam or embankment.

Overturn: The almost spontaneous mixing of all layers of water in a reservoir or lake when the water temperature becomes similar from top to bottom. This may occur in the fall/winter when the surface waters cool to the same temperature as the bottom waters and also in the spring when the surface waters warms after the ice melts.

Overwinding: A rope or cable wound and attached so that it stretches from the top of a drum to the load.

Oxbow channel: A natural U-shaped channel in a river as viewed from above.

Ozonation: The application of ozone to water for disinfection or for taste and odor control.

Ozone: A gas that is bubbled through water to kill germs.

Pacific flyway: An established air route of migratory birds along the west coast of the United States.

Paleo- Refers to the studies of the geolgic past.

Paleoflood: Paleoflood peak discharges are estimated using geology, fluvial, geomorphology, and stratigraphic records. Geologic information is used to determine flood depths, carbon 14 dating techniques are typically used to determine the time frame when these depths were reached, and hydraulic models, such as step backwater techniques, are used to determine the associated flow given the depth. These floods are used to extend gage records. Floods that have happened approximately 200- to 10,000-years ago can be estimated.

Paleoflood data: Paleoflood data include two broad categories: fluvial geomorphic evidence and botanical evidence. Paleoflood data are distinguished from both historical and systematic (conventional) flood data by lack of human observation, regardless of the time of occurrence.

Paleoflood hydrology: The study of past or ancient floods which occurred prior to the time of human observation or direct measurement by modern hydrologic procedures. The study of the movements of water and sediment in channels before the time of continuous hydrologic records or direct measurements.

Palustrine habitat: Marsh habitat.

Pan evaporation: Evaporative water losses from a standardized pan. Pan evaporation is sometimes used to estimate crop evapotranspiration and assist in irrigation scheduling.

Paradox gate: Similar to a ring follower gate except the gate leaf is supported along either side by endless trains of rollers. The gate seals by the rollers disengaging from support of the leaf when the gate is completely closed, allowing hydrostatic forces to seal the gate.

Parapet wall: A solid wall built along the upstream and/or downstream edge of the top of dam designed for ornamentation, to reflect waves, and/or for the safety of vehicles and pedestrians.

Parshall Flume (Improved Venturi Flume): A flume with a specially shaped open-channel flow section that may be installed in a drainage lateral or ditch to measure the rate of flow of water. A calibrated device consisting of a broad and flat converging section, a narrow downward sloping throat, and a diverging upward sloping section developed to measure a wide range of flows in an open channel. A calibrated device, based on the principle of critical flow, used to measure the flow of water in open channels. See short-throated flume.

Particle acceleration: The time rate of change of particle velocity.

Particle displacement: The difference between the initial position of a soil particle and any later temporary position during shaking.

Particle size: Diameter of the various particles comprising a particular soil.

Particle velocity: The time rate of change of particle displacement.

Parts per billion (ppb): One ppb is equivalent to one drop of water in 55,000 gallons.

Parts per million (ppm): Equivalent to milligrams per liter. One ppm is comparable to one drop of water in 55 gallons.

Pass: A working trip or passage of an excavating, grading, or compaction machine. See compaction.

Pattern cracking: Fine cracks in the form of a pattern on a concrete surface.

Pay formation: A layer or deposit of soil or rock whose value is sufficient to justify excavation.

Payline: Lines of excavation, backfill, compacted backfill or embankment which are described in the specifications or shown on the drawings which describe or show the limits to which earthwork is paid for.

Pea gravel: A uniformly graded gravel with a particle size of approximately 3/16".

Peak demand (peak load): Maximum electrical demand occurring within a specified period of time. See energy.

Peak flow: Maximum instantaneous flow in a specified period of time.

Peak ground acceleration (PGA) That acceleration representing the peak acceleration of ground motion.

Peak load plant: Powerplant that normally is operated to provide power during maximum load periods.

Peaking capacity: Power generating equipment operated only during highest daily, weekly, or seasonal loads. See capacity.

Peaking power: Powerplant capacity typically used to meet the highest levels of demand in a utility's load or demand profile.

Peat: A fibrous mass of organic matter in various stages of decomposition, generally dark brown to black in color and of spongy consistency. A soft light swamp soil consisting mostly of decayed vegetation. See humus.

Penstock: A pipeline or conduit designed to withstand pressure surges leading from a forebay or reservoir to power-producing turbines, or pump units. Conduit used to convey water under pressure to the turbines of a hydroelectric plant.

Per capita use: The average amount of water used per person during a standard time period, generally per day.

Percolation: Downward movement of water through the soil profile or other porous media. Water soaking into the ground. See seepage.

Percolation rate: The rate at which water moves through porous media, such as soil. The intake rate used for designing wastewater absorption systems.

Perched water table: Underground water lying over dry soil, and sealed from it by an impervious layer. A water table usually of limited area maintained above the normal free water elevation by the presence of an intervening relatively impervious confining stratum.

Perennial stream: A stream that flows continually throughout the year.

Perennial yield: Maximum quantity of water that can be annually withdrawn from a ground water basin over a long period of time (during which water supply condditions approximate average conditions) without developing an overdraft condition.

Perforated pipe: Pipe designed to discharge water through small, multiple, closely spaced orifices or nozzles, placed in a segment of its circumference for irrigation purposes.

Periodic facility review: A field review performed on a high- or significant-hazard dam every 6 years that entails a thorough examination from both operation and maintenance and dam safety perspectives. An examination on dams generally without the involvement of a senior dam engineer. The regional office has primary lead responsibility for these reviews. Periodic facility reviews are generally followed every 3 years by a comprehensive facility review.

Periphyton: Microscopic plants and animals that are firmly attached to solid surfaces under water such as rocks, logs, pilings and other structures.

Permafrost: Perennially frozen soil.

Permanent wilting point (permanent wilting percentage): Soil water content below which plants cannot readily obtain water and permanently wilt. See wilting point.

Permeability: The measure of the flow of water through soil. The ease (or measurable rate) with which gasses, liquids, or plant roots penetrate or pass through a layer of soil or porous media. The capacity or ability of a porous rock, sediment, or soil to allow the movement of water through its pores.

Permeable: Having pores or openings that permit liquids or gasses to pass through.

Permeameter: Device for containing the soil sample and subjecting it to fluid flow in order to measure permeability or hydraulic conductivity.

Pervious: Permeable, having openings that allow water to pass through.

Pervious zone: A part of the cross section of an embankment dam comprising material of high permeability.

Petcock: A small drain valve. A small ground key type valve used with soft copper tubing.

pH: Percentage of free hydrogen ions. A measurement of soil acidity. A relative scale, from 0 to 14, of how acidic or basic (alkaline) a material is, where a pH of 7 is neutral, smaller readings increasingly acid. Indicator of acidity. An expression of the intensity of the basic or acid condition of a liquid.

Philadelphia rod: A leveling rod in which the hundredths of feet, or eights of inches, are marked by alternate bars of color the width of the measurement.

Photogrammetry: Techniques of obtaining precise measurements from images. This process can derive elevation from specially created stereo pairs of aerial photos.

Photosynthesis: Process in which chlorophyll-containing cells convert light into chemical energy, forming organic compounds from inorganic compounds.

Phreatic surface: The free surface of water seeping at atmospheric pressure through soil or rock.

Phreatophyte: Water-loving plant. Vegetation that consumes much water.

Physiography: Description of nature or natural phenomenon in general.

Phytophagous: Plant eating.

Phytoplankton: Small, usually microscopic plants (such as algae), found in lakes, reservoirs, and other bodies of water.

Piezometer: An instrument which measures pressure head or hydraulic pressures in a conduit or hydraulic pressures within the fill of an earth dam or the abutment; at the foundation because of seepage or soil compression; or on a flow surface of a spillway, gate, or valve.

Piezometric surface: The surface at which water will stand in a series of piezometers.

Pig: An air manifold having a number of pipes which distribute compressed air coming through a single large line.

Pile: Relatively slender structural element which is driven, or otherwise introduced, into the soil, usually for the purpose of providing vertical or lateral support.

Pioneering: The first working over of rough or overgrown areas.

Pipe: A circular conduit constructed of any one of a number of materials that conveys water by gravity or under pressure. A cylindrical conduit or conductor, the wall thickness of which is sufficient to receive a standard pipe thread. See crown, flexible pipe, haunches, invert, rigid pipe, and springline.

Pipe trench: A temporary excavation in which pipe is placed and eventually covered with soil material.

Piping: The erosion of embankment or foundation material (soil) due to leakage. The action of water passing through or under an embankment dam and carrying with it to the surface at the downstream face some of the finer material. The progressive removal of soil particles from a mass by percolating water leading to the development of channels. The progressive development of internal erosion by seepage, appearing downstream as a hole discharging water. The process of conveying erodible embankment or foundation materials through a continuous, open "pipe" which is able to maintain a self-supported roof. The pipe normally begins at an unprotected exit and works it's way upstream (up gradient) along an erodible flow path until the reservoir is reached.

Pit: Any mine, quarry, or excavation area worked by the open-cut method to obtain material of value.

Pit run gravel: Natural gravelly material taken from excavation. Gravelly material which is not processed.

Pitching: Squared masonry, precast blocks, or embedded stones laid in regular fashion with dry or filled joints on the upstream slope of an embankment dam or on a reservoir shore or on the sides of a channel as a protection against wave and ice action.

Placed: To put in a particular place or in a particular state.

Planimeter: A device that measures area.

Plankton: Tiny plants (phytoplankton)and animals (zooplankton) with limited powers of locomotion usually living free in the water away from substrates. Minute plants and animals floating in bodies of water; often a major source of nutrition for larger aquatic life forms.

Planning basis: Guidance concerning: (1) size of planning area (distance); (2) time dependence of releases; and (3) flood characteristics of releases.

Plant: Station where mechanical, chemical, and/or nuclear energy is converted into electric energy.

Plant factor: Ratio of average load on the plant for period of time considered to be the aggregate rating of all the generating equipment installed in the plant.

Plastic limit: The water content corresponding to an arbitrary limit between the plastic and the semisolid state of consistency of a soil; water content at which a soil will just begin to crumble when rolled into a thread approximately 1/8 inches in diameter. The minimum amount of water in terms of percent of oven-dry weight of soil that will make the soil plastic.

Plastic soil: A soil that exhibits plasticity.

Plasticity: The property of a soil or rock which allows it to be deformed beyond the point of recovery without cracking or appreciable volume change.

Plasticity index: Numerical difference between the liquid limit and the plastic limit.

Playa: An intermittent shallow lake formed at the bottom of an undrained desert basin after heavy rains.

Plot: Area of land that is studied or used for an experimental purpose, in which sample areas are often located.

Plumb: Exactly vertical; at right angles to the horizontal.

Plumb Bob: A pointed weight hung from a string. Used for vertical alignment.

Plunge pool: A natural or artifically created pool that dissipates the energy of free-falling water.

Ply: One of several layers of fabric or of other strength-contributing material.

Pneumatic: Powered or inflated by compressed air.

Point of intersection (PI):

Point source: Any discernible, confined, or discrete conveyance from which pollutants are or may be discharged, including, but not limited to, any pipe, ditch, channel, tunnel, conduit, well, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft. A stationery location or fixed facility from which pollutants are discharged or emitted. Any single identifiable source of pollution. Pollution that comes from a well-defined source.

Pollutant: Any inorganic or organic substance that contaminates air, water or soil.

Pollution: A resource that is out of place.

Polyvinyl Chloride (PVC): A type of plastic used to make pipe and fittings for water distribution and irrigation.

Pond: A small lake. Water impounded by a diversion dam.

Ponding (or flooding): A method for compacting soil using water added after the soil is in place until free water stands on the surface.

Pool: A deep area of a stream between rapids or where the current is slow.

Population: Total of individuals occupying an area.

Population at risk (PAR): The population potentially affected by flood waters as a result of large operational releases or dam failure. The potential number of persons whose lives are at risk in the event of a dam failure.

Population density: Number per unit area of individuals of any given species at a given time.

Pore pressure: The interstitial pressure of fluid (air or water) within a mass of soil, rock, or concrete.

Pore spaces: The open areas, or spaces, in soil, sediments, and rocks that are filled by air or water.

Pore-water pressure: Internal hydrostatic pressure in an embankment caused by the level of water in the reservoir acting through pressure-transmitting paths between soil particles in the fill.

Porosity: The ratio of the volume of void space to the total volume of an undisturbed sample. A measure of the ratio of open space within a rock or soil to its total volume. A nondimensional value that expresses the ratio of the volume of pores to the total volume of a porous material and is usually expressed as a percentage. Porosity ranges from less than 1 percent to as much as 80 percent in some recently deposited clays, but in most granular materials it falls between about 5 and 40 percent. In free aquifers the porosity is equal to the specific retention plus the specific yield. The capacity of soil or rock to hold water.

Portal: An opening into a tunnel.

Positive pressure: Pressure within a pipe that is greater than atmospheric pressure.

Posted operating instructions: Operations and maintenance instructions taken from the Standing Operating Procedures (SOP) that pertain to the mechanical features in the immediate area of the dam and which are posted adjacent to those features.

Potable water: Water that is safe and satisfactory for drinking and cooking.

Potential dam safety deficiency: A condition that currently does not significantly affect the safety of the dam, but is capable of becoming a dam safety deficiency; for example, continuing erosion, tree growth, or a potential adverse response of the dam to an unusual loading condition such as a PMF or MCE. A potential dam safety deficiency is considered to exist when critical analysis parameters are unknown such that additional investigations and studies are needed to conclusively determine whether or not a dam safety deficiency actually exists.

Potential evapotranspiration: Rate at which water, if available, would be removed from soil and plant surfaces.

Potential spill: An accident or other circumstances which threatens to result in discharge of oil or other hazardous substances.

Pounds per square inch (psi or lb/in2): A pressure designation for pounds per square inch. May be pounds per square inch gage (psig) or absolute (psia). Psig measures pressure above the local atmospheric pressure. Psia measures pressure with absolute vaccuum as a reference.

Powder factor (explosives factor): A term expressing the amount of explosives used to break a quantity of rock. Normal units for the powder factor are lb/yd3 or kg/m3; however, lb/ton and kg/ton are also used as well as the reciprocals of all of these terms.

Power: Electrical energy generated, transferred, or used; usually expressed in kilowatts.

Power demand: Rate at which electric energy is required and delivered to or by a system over any designated period of time.

Powerplant: Structure that houses turbines, generators, and associated control equipment.

Powerplant capacity: The capacity for powerplants is the nameplate rating in kW (kilowatts) and generally includes only the main generating units, except for very large plants such as Grand Coulee and Hoover where the station service units are included in the total rated capacity. Maximum flow that can pass through the turbines at a dam. Also refers to the electrical capacity of the generators expressed in megawatts.

Power pool: Two or more interconnected electric systems which operate as a single system to supply power to meet combined load requirements.

Precast dam: A dam constructed mainly of large precast concrete blocks or sections.

Precipitation: The total measurable amount of water received in the form of snow, rain, drizzle, hail, and sleet.

Predominant period: The period(s) at which maximum spectral energy is concentrated.

Preference: Priority access to Federal power by public bodies and cooperatives.

Preference customers: Publicly owned systems and non-profit cooperatives which, by law, have preference over investor-owned systems for purchase of power from Federal projects.

Prescribed elevation: The elevation as called out or dictated in notes, drawings or specifications paragraphs.

Pressure head: The amount of force or pressure created by a depth of one foot of water.

Pressure relief pipes: Pipes used to relieve uplift or pore-water pressure in a dam foundation or in the dam structure.

Prestressed dam: A dam, the stability of which depends in part on the tension in steel wires, cables, or rods that pass through the dam and are anchored into the foundation rock. See Stewart Mountain Dam.

Primary excavation: Digging in undisturbed soil, as distinguished from rehandling stockpiles.

Primary production: Creation of organic matter by photosynthesis.

Probability: The likelihood of an event occurring.

Probability of failure: Likelihood that, given the loading event and failure mode, the structure responds with the necessary adverse occurrences to ultimately result in uncontrolled reservoir release.

Probability of load: Likelihood that a certain loading event (static, seismic, hydrologic, other) occurs at a structure site.

Probable maximum flood (PMF) (maximum probable flood, MPF): The largest flood that may reasonably be expected to occur at a given point on a stream from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible on a particular watershed. This term identifies estimates of hypothetical flood characteristics (peak discharge, volume, and hydrograph shape) that are considered to be the most severe "reasonably possible" at a particular location, based on relatively comprehensive hydrometeorological analyses of critical runoff-producing precipitation (and snowmelt, if pertinent) and hydrologic factors favorable for maximum flood runoff. The maximum runoff condition resulting from the most severe combination of hydrologic and meteorologic conditions that are considered reasonably possible for the drainage basin under study.

Probable maximum precipitation (PMP): Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographic location at a certain time of the year.

Production: Act or process of producing electrical energy from other forms of energy; also, amount of electrical energy produced expressed in kilowatt-hours (kWh).

Profile: A sectional view showing grades and distances, usually taken along the centerline.

Profundal: Deepest part of the ocean or lake where light does not penetrate. This layer usually has fewer nutrients, more silt, and fewer organisms than the surface.

Programmatic environmental impact statement (PEIS): An environmental impact statement that addresses a proposal to implement a specific policy, to adopt a plan for a group of related actions, or to implement a specific statutory program.

Proratable water: That portion of the total water supply available under provisions of sections 18 and 19 of Civil Action No. 21 (Federal District Court Judgment of January 31, 1945) that is subject to proration in times of water shortage.

Protective action decision making (recommendations): The process whereby local authorities select one of more actions to protect threatened populations. Personnel of the dam operating organization will make recommendations as part of the accident assessment and emergency classification level system process.

Psychological value. See Social value.

P.T: Pipe thread.

Public affairs officer: The dam operating organization's designated person responsible for public affairs. The public affairs officer is the dam operating organization's counterpart to the downstream jurisdiction's public information officer.

Public information officer (public relations officer): The person on the emergency management team who is in charge of public information affairs.

Public involvement: Process of obtaining citizen input into each stage of development of planning documents. Citizen input is required in all environmental impact statements.

Public involvement plan: Document that presents the procedural plans of the Bureau of Reclamation to inform and gather information from project beneficiaries and the general public.

Publics: Any interested group or individual, including Federal, State and local agencies, interest groups, ad hoc groups, and the general public.

Puddle: To compact loose soil by soaking it and allowing it to dry.

Puddling: A method of compacting soil with the soil deposited into a pool of water and then stirred or rodded.

Pull shovel (dragshovel or hoe): A shovel with a hinge-and-stick mounted bucket that digs while being pulled inward.

Pump: See centrifugal pump, diaphragm pump, jetting pump.

Pump-back system: A return-flow system in which tailwater is pumped back to the head of an irrigation ditch for reuse.

Pumped-storage plant: Powerplant designed to generate electric energy for peakload use by releasing water previously pumped into an elevated storage reservoir, usually during off-peak periods.

Pumping: Mechanical transfer of fluids.

Pumping energy: Energy required to pump water from the lower reservoir to the upper reservoir of a pumped-storage plant.

Pumping plant: Facility that lifts water up and over hills.

Pumplift (pumping lift): The vertical distance that a pump will raise waters. Distance water must be lifted in a well from the pumping level to the ground surface.

Quadrat: Sampling area, most commonly one square meter, used for analyzing vegetation. A square or rectangular plot of land marked off for the study of plants and animals.

Quantitative precipitation forecasts: A projection that uses the interpretation of various observational, numerical, and objective aids to estimate the amount, location, and duration of precipitation. The precipitation forecasts are used as a decision making tool for flood warning systems in basins subject to flash floods.

Quarry: A rock pit. An open cut mine in rock chosen for physical rather than chemical characteristics.

Quaternary: Refers to the geologic period of the last 2,000,000 years and includes the Pleistocene and Holocene (last 10,000 years) epochs.

Quicksand: Fine sand or silt that is prevented from settling firmly together by upward movement of ground water. Any wet inorganic soil so unsubstantial that it will not support any load.

Radial: Lines converging at a single center.

Radial gate: A pivoted crest gate, the face of which is usually a circular arc, with the center of curvature at the pivot about which the gate swings. A gate with a curved upstream plate and radial arms hinged to piers or other supporting structure. See tainter gate, or monocoque gate.

Radius: Horizontal distance from the center of rotation.

Raffinate ponds: Liquid resulting from extractions with a solvent.

Ramp flume: A flume calibrated to measure the flow of liquid in an open channel.

Ramp rate: The rate of change in instantaneous output from a powerplant. The ramp rate is established to prevent undesirable effects due to rapid changes in loading or discharge.

Random earthquake: In addition to earthquakes that occur on mapped faults, hazards are also presented by earthquakes not associated with known geological structures. The occurrence of these events is best viewed as a random process, hence the term "random" earthquake. Based on earthquake recurrence relationships for an area, estimates of source-to-site distances can be calculated for various magnitudes and probabilities of occurrence.

Range: Geographic region in which a given plant or animal normally lives or grows.

Rapid: A section of a river where the current is very fast moving, caused by a steep descent in the riverbed through a constriction of the main channel.

Rapid flow: Also refered to as supercritical flow, rapid flow is distinguished from tranquil flow by a dimensionless number called the Froude number. If the Froude number is less than one, the flow is tranquil. If the Froude number is greater than one, the flow is rapid. If the Froude number is equal to one, the flow is critical. Surface waves can propagate only in the downstream direction. Control of rapid flow depth is always at the upstream end of the rapid flow region.

Raptors: Birds of prey.

Rated head: Water depth for which a hydroelectric generator and turbines were designed.

Ratio of reduction: The relationship between the maximum size of the stone which will enter a crusher, and the size of its product.

Reach: Any specified length of stream, channel, or other water conveyance. A portion of a stream or a river. The area of a canal or lateral between check structures. Sometimes also used to describe a contiguous stretch of river.

Reattachment deposit: Sand deposit located where downstream flow meets the channel bank at the downstream end of a recirculating zone.

Rebar: Reinforcing steel bar. Reinforcement.

Recharge: Increases in ground water storage due to precipitation, infiltration from streams, or human activity.

Recirculation zone: Area of flow composed of one or more eddies immediately downstream from a constriction in the channel, such as a debris fan or rock outcrop.

Reclamation Reform Act of 1982 (RRA): Requires districts that have certain repayment or water service contracts with the United States to develop water conservation plans which include definite goals, appropriate water conservation measures, and a time schedule for meeting the water conservation objectives.

Recreational benefit: Value of recreational activity to the recreationist, usually measured in dollars above the cost of participating in the recreational activity (travel, entrance fees, etc). Used for valuing recreational resources produced through Federal projects, synonymous with the consumer surplus associated with the recreational activity.

Recruitment: Survival of young plants and animals from birth to a life stage less vulnerable to environmental change.

Rectangular weir: A contracted or suppressed weir with a horizontal crest, rectangular in shape, having vertical sides. See weir.

Redd: The nest that a spawning female salmon digs in gravel to deposit her eggs. Depression in riverbed or lakebed dug by fish to deposit eggs.

Refill: Material which had previously been excavated as a result of construction activities and again placed to the lines as shown on the drawings.

Refuge compatibility requirements: Requirements under the Refuge Administration Act that all uses of a national wildlife refuge must be compatible with the purpose for which the refuge was established.

Regional Response Team (RRT): Regional planning and coordination of preparedness and response actions are accomplished through the RRT. RRT membership is the same as the National Response Team (NRT) but also includes state representation through the SERC. RRT may also include incident - specific teams established following notification of an incident through membership of the RRT as they relate to the technical nature and geographic location of the incident.

Regulating dam (reregulating dam): A dam impounding a reservoir from which water is released to regulate the flow downstream. See afterbay dam.

Regulating gate (operating gate, regulating valve): A gate used to regulate the rate of flow through an outlet works or spillway. A gate or valve that operates under full pressure flow conditions to regulate the rate of discharge.

Rehabilitation: The process of renovating a facility or system whose performance is failing to meet the original criteria and needs of the project.

Reimbursable: Costs of constructing, operating, or maintaining a Reclamation project that are repaid to the Federal Government by some other individual, entity, or organization.

Relative density: Used in construction control for cohesionless soils where the in- place density is compared to the minimum and maximum density of the soil from laboratory tests. The ratio of the difference between the void ratio of a cohesionless soil in the loosest state and any given void ratio to the difference between its void ratios in the loosest and in the densest states.

Relative humidity: The ratio of the amount of moisture in the air to the maximum amount of moisture the air could hold under the same conditions; usually expressed as a percentage.

Release: The amount of water released after use. The difference between delivery and release is usually the same as consumptive use.

Reliability: Probability that a device will function without failure over a specified time period or amount of usage.

Relict: A species, population, etc., which is a survivor of a nearly extinct group. Any species surviving in a small local area and widely separated from closely related species.

Relief: Term designating the path of least resistance through which energy from explosions can be released. This path is usually taken to a free face or surface where rock can displace and energy can be released.

Relief valve: A valve which will allow air or fluid to escape if its pressure becomes higher than the valve setting. A safety device that automatically provides protection against excessive temperatures, excessive pressures, or both.

Relief well. See drainage well.

Remote operation: Operation of mechanical features from an on-site location other than at the feature.

Remote sensing: Method for determining characteristics of an object, organism or community from afar.

Reportable discharge of oil (40 CFR 110.3): Harmful quantities of oil discharged or released to navigable waters of the United States are any amount that violates State water quality standards and/or causes a sheen or film (as little as 10 parts per million of oil can cause a sheen), upon or discoloration of the surface of the water or adjoining shorelines or cause a sludge or emulsion to be deposited beneath the surface of the water or upon adjoining shorelines. An exception is a sheen caused by outboard motors.

Reportable quantities: Quantities of hazardous substances which require notification to the National Response Center and the appropriate state agency.

Reregulation dam. See afterbay dam.

Reserve generating capacity: Extra generating capacity available to meet unanticipated capacity demand for power in the event of generation loss due to scheduled or unscheduled outages of regularly used generating capacity.

Reserved works: Facilities operated and maintained by Reclamation.

Reservoir: A body of water impounded by a dam and in which water can be stored. Artificially impounded body of water. Any natural or artificial holding area used to store, regulate, or control water. Body of water, such as a natural or constructed lake, in which water is collected and stored for use. Dam design and reservoir operation utilize reservoir capacity and water surface elevation data. To ensure uniformity in the establishment, use, and publication of these data, the following standard definitions of water surface elevations shall be used. See maximum controllable water surface, maximum water surface, normal water surface, top of active conservation capacity, top of dead capacity, top of exclusive flood control capacity, top of inactive capacity, and top of joint use capacity.

Reservoir capacity: The capacity of the reservoir, usually in acre-feet. Dam design and reservoir operation utilize reservoir capacity and water surface elevation data. To ensure uniformity in the establishment, use, and publication of these data, the following standard definitions of reservoir capacities shall be used. Reservoir capacity as used here is exclusive of bank storage capacity. See active capacity, active conservation capacity, dead capacity, exclusive flood control capacity, flood control capacity, inactive capacity, joint use capacity, live capacity, and total capacity.

Reservoir capacity allocations (RCA) sheet: A summary of acre-feet allocations of water, to such purposes as surcharge, exclusive flood control capacity, joint use capacity, active conservation capacity, inactive capacity, and dead capacity.

Reservoir inflow: The amount of water entering a reservoir expressed in acre-feet per day or cubic feet per second.

Reservoir regulation (or operating) procedure: Operating procedures that govern reservoir storage and releases.

Reservoir surface area: The area covered by a reservoir when filled to a specified level.

Resilience: Ability of any system to resist or to recover from stress or hardship.

Response spectrum: A plot of the maximum response of a series of single-degree-of-freedom damped oscillators (elastic systems) as a function of their natural periods, or frequencies, when the oscillators are subjected to a vibratory ground motion. The maximum values of acceleration, velocity, and/or displacement of an infinite series of single-degree-of-freedom system subjected to an earthquake. The maximum response values are expressed as a function of natural period for a given damping. The response spectrum acceleration, velocity, and displacement values may be calculated from each other assuming a sinusoidal relationship between them.

Retaining wall: A wall separating two levels.

Return flow: Drainage water from irrigated farmlands that re-enters the water system to be used further downstream. May contain dissolved salts or other materials that have been leached out of the upper layers of the soil. That portion of the water previously diverted from a stream which finds its way back to that stream or to another body of ground or surface water. The water that reaches a ground or surface water source after release from the point of use and thus becomes available for further use.

Return-flow system (reuse system): A system of pipelines or ditches to collect and convey surface or subsurface runoff from an irrigated field for reuse.

Revetment: An embankment or wall of sandbags, earth, etc., constructed to restrain material from being transported away. A facing of stone, cement, sandbags, etc., to protect a wall or embankment.

Review of Operation and Maintenance (RO&M): The RO&M program recognizes the following three categories for reporting deficiencies:

Category 1 - Recommendations involving the correction of severe deficiencies where immediate and responsive action is required to ensure structural safety and operational integrity of a facility.

Category 2 - Recommendations covering a wide range of important matters where action is needed to prevent or reduce further damage or preclude possible operational failure of a facility.

Category 3 - Recommendations covering less important matters but believed to be sound and beneficial suggestions to improve or enhance the operations and maintenance (O&M) of the project or facility.

Rewind: Act of putting new copper insulated wire in the armature windings of a generator.

Richter scale: A scale of numerical values, proposed by C. F. Richter, to describe the magnitude of an earthquake, ranging from 1 to 9, by measurements made in well-defined conditions and with a given type of seismograph. The zero of the scale is fixed arbitrarily to fit the smallest recorded earthquake. The largest recorded earthquake magnitudes are near 8.7 This is the result of observations and not an arbitrary upper limit like that of the intensity scale. For more information about the Richter Scale visit the U.S. Geological Survey National Earthquake Information Center.

Riffle: A stretch of choppy water caused by an underlying rock shoal or sandbar. A run is a long riffle.

Riffle and pool complex: A water habitat composed of riffles (characterized by water flowing rapidly over a coarse substrate) and pools (deeper areas of water associated with riffles).

Right: See left or right designation.

Right abutment: See abutment.

Right-of-way: The land on and immediately surrounding a structure that is owned by the entity holding title to the structure.

Rigid pipe: Pipe designed to transmit the backfill load to the foundation beneath the pipe. Rigid pipe must be supported on the bottom portion of the pipe.

Rill: A small channel eroded into the soil surface by runoff, rills easily can be smoothed out (obliterated) by normal tillage. Small grooves, furrows, or channels in soil made by water flowing down over its surface. A small stream.

Ring-follower gate: A gate consisting of a rectangular leaf and an opening equal in diameter to that of the conduit that forms an unobstructed passageway when the leaf is in the raised or open position. A high pressure gate of the vertical lift type with a circular water passage. A corresponding circular passage is incorporated into half of the gate leaf which provides an unbroken flow surface in the waterway when the gate is completely open.

Ring gate: A ring- or annular-shaped steel drum operating in a recess or gate chamber in a spillway crest and controlled in a manner similar to a drum gate. See gate.

Ring seal gate: Similar to a paradox gate, however sealing is by a moveable seal, which is extended by water pressure when the gate is closed. The roller chain is usually shorter and does not disengage from the leaf on sealing.

Riparian: Living on or adjacent to a water supply such as a riverbank, lake, or pond. Of, on, or pertaining to the bank of a river, pond, or lake.

Ripple: In a streambed, a ripple is a small triangle-shaped elements having gentle upstream slopes and steep downstream slopes.

Riprap: A layer of large uncoursed stones, broken rock, boulders, precast blocks, bags of cement, or other suitable material generally placed in random fashion on the upstream and downstream faces of embankment dams, stream banks, on a reservoir shore, on the sides of a channel, or other land surfaces to protect them from erosion or scour caused by current, wind, wave, and/or ice action. A protective blanket of large loose stones, which are usually placed by machine to achieve a desired configuration. Riprap is usually placed by dumping or other mechanical methods but, in some cases, is hand placed. It consist of relatively large pieces as distinguished from a gravel blanket. Very large riprap is sometimes referred to as "armoring."

Risk: The relationship between the consequences resulting from an adverse event and its probability of occurrence. The potential for losing credibility, failing to solve a problem, or getting hurt.

Risk assessment: As applied to dam safety, the process of identifying the likelihood and consequences of dam failure to provide the basis for informed decisions.

River corridor: A river and the strips of land adjacent to it, including the talus slopes at the bases of cliffs, but not the cliffs themselves.

River mile (RM): A unit of measurement (in miles) used on rivers.

River outlet works (row): See outlet works.

River trash wall: Walls constructed to deflect heavy floating debris away from the upper ends of a fishway.

Riverine: Riparian; pertaining to a riverbank.

Rock: The hard, firm and stable parts of the earth's crust. A sound and solid mass, layer, or ledge of mineral matter inplace, and of such hardness and texture that it cannot be effectively loosened or broken down by a 6-pound drifting pick or by ripping in a single pass. Natural solid mineral matter occurring in large masses or fragments.

Rock anchor: A steel rod or cable placed in a hole drilled in rock, held in position by grout, mechanical means, or both. In principle, the same as a rock bolt, but usually the rock anchor is more than 4 meters long.

Rock bolt: A steel rod placed in a hole drilled in rock, held in position by grout, mechanical means, or both. A rock bolt can be pretensioned.

Rock excavation: Hard and firm parts of the earth's crust which is dug out and removed from a particular site or area, see rock. Boulders or detached pieces of solid rock more than 1 cubic yard in volume are classified as rock excavation. See excavation.

Rockfill dam: An embankment dam in which more than 50 percent of the total volume is comprised of compacted or dumped cobbles, boulders, rock fragments, or quarried rock generally larger than 3-inch size. The rock provides structural integrity for the dam around an impervious core.

Rock fragment: Detached pieces of rock which generally are not rounded.

Rolled fill dam: An embankment dam of earth or rock in which the material is placed in layers and compacted by the use of rollers or rolling equipment.

Roller compacted concrete (RCC): A mixture of cement, water, and aggregate compacted by rolling.

Roller-compacted concrete dam: A concrete gravity dam constructed by the use of a dry mix concrete transported by conventional construction equipment and compacted by rolling, usually with vibratory rollers.

Roller drum gate. See drum gate.

Roller gate: A gate rolled up or down inclined supporting rails by a hoist through sprocket chains around the ends of a cylinder. A crest gate consisting of a horizontal cylinder spanning between piers. Encircling each end of the cylinder is a cog wheel which engages a rack in each supporting pier. The gate is raised by a heavy chain or rope which winds around and over the top of the gate at one end and pulls upward parallel to the track.

Rolling: A method of compacting soil using a sheepsfoot roller or a smooth drum roller.

Root zone: That depth of soil which plant roots readily penetrate and in which the predominant root activity occurs. The area where a low-angle thrust fault steepens and descends into the crust.

Rough fish: A nonsport fish.

Route alerting: A supplement to the public alert system; a method for alerting people in areas not covered by the primary system or if the primary system fails. Route alerting is accomplished by emergency personnel in vehicles traveling along assigned roads and delivering emergency instructions with public address systems or by door-to-door notification.

Rubble dam: A stone masonry dam in which the stones are unshaped or uncoursed. See dam.

Run: Seasonal upstream migration of anadromous fish. One or more lengths of pipe that continue in a straight line.

Runoff: The portion of precipitation, snow melt, or irrigation that flows over the soil, eventually making its way to surface water supplies. Liquid water that travels over the surface of the Earth, moving downward due to the law of gravity; runoff is one way in which water that falls as precipitation returns to the ocean.

Run-of-river plants: The regulated inflow of one powerplant is equal to the outflow from a powerplant upstream.

Runup: The vertical distance above the setup that the rush of water reaches when a wave breaks on the dam embankment.

Sacred site: Any specific, discrete, narrowly delineated location on Federal land that is identified by an Indian Tribe, or Indian individual determined to be an appropriately authoritative representative of an Indian religion, as sacred.

Saddle dam: A subsidiary dam of any type constructed across a saddle or low point on the perimeter of a reservoir. See dike.

Safe Drinking Water Act (SDWA): Passed in 1974, SDWA is the main federal law that ensures the quality of Americans' drinking water.

Safe yield: The annual quantity of water that can be taken from a source of supply over a period of years without depleting the source beyond its ability to be replenished naturally in wet years.

Safety evaluation flood (SEF): The largest flood for which the safety of a dam and appurtenant structure is to be evaluated.

Safety Evaluation of Existing Dams (SEED) examination: The onsite examination performed initially and at predetermined intervals (approximately every 6 years). The design, construction, operation, performance, and existing condition of all features are evaluated in accordance with state-of-the-art criteria. See overall safety of dams classification.

Safety Evaluation of Existing Dams (SEED) report: A compilation of independent technical reports that evaluate the design, construction, and performance of a dam for its structural and hydraulic integrity using available data; identify existing or potential dam safety deficiencies; and recommend future actions appropriate for the safety of the dam. Evaluation includes review of hydrology, geology, seismicity, seepage, structural adequacy, design criteria, construction, operation, instrumentation records, existing field conditions, and past performance. See evaluation report. See overall safety of dams classification.

Saline: The condition of containing dissolved or soluble salts. Saline soils are those whose productivity is impaired by high soluble salt content. Saline water is that which would impair production if used to irrigate sensitive crops without adequate leaching to prevent soil salinization.

Saline sodic land: Soil that contains soluble salts in amounts that impair plant growth but not an excess of exchangeable sodium.

Saline water: Water that contains more than 1,000 milligrams per liter of dissolved solids.

Salinity: Saltiness. The relative concentration of dissolved salts, usually sodium chloride, in a given water supply. A measure of the concentration of dissolved mineral substances in water.

Salmonids: Family of fish that includes salmon and steelhead.

Saltation: The movement of sand or fine sediment by short jumps above a streambed under the influence of a water current too weak to keep it permanently suspended in the moving water.

Sample error: Random variation reflecting the inherent variability within a population being counted.

Sand: Mineral grains whose particle size vary from a No. 4 sieve to a No. 200 sieve. A loose soil composed of particles between 1/16 mm and 2 mm in diameter.

Sand backfill: Material which has a particle size which varies from a No. 4 sieve to a No. 200 sieve and is used for refilling an excavation.

Sand boil: Seepage characterized by a boiling action at the surface surrounded by a cone of material from deposition of foundation and/or embankment material carried by the seepage. A swirling upheaval of sand or soil on the surface of or downstream from an embankment caused by water leaking through the embankment. The ejection of sand and water resulting from piping.

SARA Title III: Superfund Amendments and Reauthorization Act of 1986 known as the Emergency Planning and Community Right-to-Know Act of 1986.

Saturated zone: A zone in which all voids are filled with water. The area below the water table where all open spaces are filled with water. See ground water.

Saturation (and internal vibration): A method of compacting soil using water added to soil and internal vibrators (such as a concrete vibrator) are worked down through the depth of soil placed. The condition of being filled to capacity.

Scale: Ratio of map distance to Earth distance. Thus, in a 1:24,000 scale map, one centimeter, inch, or foot equals 24,000 centimeters, inches, or feet on the ground. Graphic scales typically show equivalent map and ground distance in the form of a line or bar.

Scaling: Prying loose pieces of rock off a face or roof to avoid danger of their falling unexpectedly. An adjustment to an earthquake time history or response spectrum where the amplitude of acceleration, velocity, and/or displacement is increased or decreased, usually without change to the frequency content of the ground motion. The earthquake time history or response spectrum can be scaled based on ground motion parameters of peak acceleration, peak velocity, peak displacement, spectrum intensity, or other appropriate parameters.

Scatter: A concentration of artifacts.

Scheduled outage: Shutdown of a generating unit, or other facility, for inspection or maintenance, in accordance with an advance schedule.

Scour: Erosion in a stream bed, particularly if caused or increased by channel changes.

Scrape: A nest made from scratching in the ground.

Screen: A mesh or bar surface used for separating pieces or particles of different sizes. A filter.

Seam: A layer of rock, coal, or ore.

Seasonal stream. See intermittent stream.

Secchi depth: A measure of water clarity.

Second foot (sec-ft): Shortened term for cubic foot per second (cfs or ft3/s).

Section: An area equal to 640 acres or 1 square mile.

Sediment: Any finely divided organic and/or mineral matter deposited by air or water in nonturbulent areas. Unconsolidated solid material that comes from weathering of rock and is carried by, suspended in, or deposited by water or wind.

Sedimentation: Deposition of waterborne sediments due to a decrease in velocity and corresponding reduction in the size and amount of sediment which can be carried.

Sediment concentration: The quantity of sediment relative to the quantity of transporting fluid, or fluid-sediment misture. The concentration may be by weight or by volume. When expressed in ppm, the concentration is always in ratio by weight.

Sediment discharge: Rate at which sediment passes a stream cross-section in a given period of time, expressed in millions of tons per day (mtd).

Sediment load: Mass of sediment passing through a stream cross section in a specified period of time, expressed in millions of tons (mt). Amount of sediment carried by running water. The sediment that is being moved by a stream.

Sediment yield: Amount of mineral or organic soil material that is in suspension, is being transported, or has been moved from its site of origin. The portion of eroded material that does travel through the drainage network to a downstream measuring or control point. The dry weight of sediment per unit volume of water-sediment mixture in place, or the ratio of the dry weight of sediment to the total weight of water-sediment mixture in a sample or a unit volume of the mixture.

Sediment yield rate: The sediment yield per unit of drainage area.

Seep: A spot where ground water oozes slowly to the surface, usually forming a pool.

Seepage: The slow movement or percolation of water through soil or rock. Movement of water through soil without formation of definite channels. The movement of water into and through the soil from unlined canals, ditches, and water storage facilities. The slow movement or percolation of water through small cracks, pores, interstices, etc., from an embankment, abutment, or foundation.

Seepage collar: A projecting collar of concrete built around the outside of a tunnel or conduit, within an embankment dam, to reduce seepage along the outer surface of the conduit.

Seepage force: The force transmitted to the soil or rock grains by seepage.

Seepage loss: Water loss by capillary action and slow percolation.

Seepage velocity: The rate of discharge of seepage water through a porous medium per unit area of void space perpendicular to the direction of the flow.

Seiche wave: A wave generated by either a landslide into a reservoir or by a sudden displacement or deformation of a fault line in a reservoir floor during a major earthquake.

Seismic: Of or related to movement in the earth's crust caused by natural relief of rock stresses.

Seismic evaluation criteria: A guideline for determining which faults or seismic sources need to be assigned MCE's. For high hazard structures, faults with Holocene or latest Pleistocene displacement are included and probabilistic assessments are based on an annual probability of occurrence of 2 x 10-5. For significant hazard structures, faults with Holocene displacement are included and probabilistic assessments are based on an annual probability of occurrence of 1 x 10-4.

Seismic intensity: Subjective measurement of the degree of shaking at a specified place by an experienced observer using a descriptive scale. See intensity scale.

Seismic parameters: Descriptors of earthquake loading or earthquake size, such as magnitude, peak acceleration, location (distance and focal depth), spectrum intensity, or any of many other parameters useful in characterizing earthquake loadings.

Seismo- Pertains to earthquakes.

Seismotectonic: Of, relating to, or designating structural features of the earth which are associated with or revealed by earthquakes.

Seismotectonic Province: A geographic area characterized by a combination of geology and seismic history.

Seize: To bind wire rope with soft wire, to prevent it from ravelling when cut.

Select material: Backfill materials specially selected and segregated from excavated materials.

Semipervious zone. See transition zone.

Sensitive species: Species not yet officially listed but undergoing status review for listing on the U.S. Fish and Wildlife Service's (FWS) official threatened and endangered list; species whose populations are small and widely dispersed or restricted to a few localities; and species whose numbers are declining so rapidly that official listing may be necessary.

Separation deposit: Sand deposit located at the upstream end of a recirculation zone, where downstream flow becomes separated from the channel bank.

Service spillway (primary spillway): A structure located on or adjacent to a storage or detention dam over or through which surplus or floodwaters which cannot be contained in the allotted storage space are passed, and at diversion dams to bypass flows exceeding those which are turned into the diversion system. Included as part of the spillway would be the intake and/or control structure, discharge channel, terminal structure, and entrance and outlet channels. A spillway that is designed to provide continuous or frequent regulated or unregulated releases from a reservoir without significant damage to either the dam or its appurtenant structures.

Settlement: The sinking of land surfaces because of subsurface compaction, usually occuring when moisture added deliberately or by nature, causes a reduction in void volumes.

Setup: The vertical rise in the stillwater level at the upstream face of a dam caused by wind stresses on the water surface.

Sewer tile: Glazed waterproof clay pipe with bell joints.

Shaft: A round bar that rotates or provides an axis of revolution. A vertical or steeply inclined tunnel.

Shaft house: House at top of the access shaft to gate chamber, which secures access to the gates.

Shaft spillway: A vertical or inclined shaft into which water spills and then is conveyed through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a bellmouth, glory hole, or morning glory spillway.

Shale: A rock formed of consolidated mud.

Shear: A structural break where differential movement has occurred along a surface or zone of failure.

Shear strength: The maximum resistance of a soil or rock to shearing stresses.

Shear wall: A vertical lateral-force-resisting element in a structure assigned to resist wind or earthquake generated lateral forces. Depending on detailing and transfer mechanisms, a shear wall can be load-bearing.

Shear zone: An area where the rock mass has moved along the plane of contact which often becomes a channel for ground water.

Sheepsfoot roller: A tamping roller having lugs with feet extending at their outer tips.

Sheet piling: Steel strips shaped to interlock with each other when driven into the ground.

Sheet wash: A flow of rainwater that covers the entire ground surface with a thin film and is not concentrated into streams.

Shelterbelt: A natural or planned barrier of trees or shrubs to reduce erosion and provide shelter from winds or storms.

Sheltering (in-place): A protective action that involves taking cover in upper levels of a building that is able to withstand high flood levels.

Shoring: Temporary bracing to hold the sides of an excavation from caving.

Shell. See shoulder.

Shelterbelt: A natural or planned barrier of trees or shrubs to reduce erosion and provide shelter from winds or storms.

Short-throated flumes: Short-throated flumes are considered short because they control flow in a region that produces curvilinear flow. While they may be termed shortthroated, the overall specified length of the finished structure, including transitions, may be relatively long. The Parshall flume is the main example of this kind of flume. These flumes would require detailed accuracy and accurate knowledge of the individual streamline curvatures for calculated ratings which is usually considered impractical. Thus short-throated flumes are determined empirically by comparison with other more precise and accurate water measuring systems.

Shoulder (shell): The upstream and downstream parts of the cross section of an embankment dam on each side of the core or core wall. Hence the expression upstream shoulder or downstream shoulder. The graded part of a road on each side of the pavement. The side of a horizontal pipe, at the level of the center line.

Shrinkage: Loss of bulk of soil when compacted in a fill.

Shrinkage index (SI): The numerical difference between the plastic and shrinkage limits.

Shrinkage limit (SL): The maximum water content at which a reduction in water content will not cause a decrease in volume of the soil mass.

Shrinkage ratio (R): The ratio of a given volume change, expressed as a percentage of the dry volume, to the corresponding change in moisture content above the shrinkage limit, expressed as a percentage of the weight of the oven-dried soil.

Significant hazard: A downstream hazard classification for dams in which 1-6 lives are in jeopardy and appreciable economic loss (rural area with notable agriculture, industry, or worksites, or outstanding natural resources) would occur as a result of failure of the dam. This classification also applies to structures other than dams.

Significant wave height: The average height of the one-third highest waves of a given wave group.

Sill: A submerged structure across a river to control the water level upstream. The crest of a spillway. The horizontal gate seating, made of wood, stone, concrete, or metal at the invert of any opening or gap in a structure. Hence, the expressions: gate sill, stoplog sill.

Silt (rock flour): The fine-grained portion of soil that is nonplastic or very slightly plastic and that exhibits little or no strength when air dry. Nonplastic soil which passes a No. 200 United States Standard sieve. A soil composed of particles between 1/256 mm and 1/16 mm in diameter. A heavy soil intermediate between clay and sand.

Silting: Filling with soil or mud deposited by water.

Simulid: Group of two-winged flying insects who live their larval stage underwater and emerge to fly about as adults.

Sinkhole: A steep-sided depression formed when removal of subsurface embankment or foundation material causes overlying material to collapse into the resulting void.

Sinuosity: The ratio of the length of a river's thalweg to the length of the valley proper. A measure of a river's meandering. Rivers with a sinuosity less than 1.5 are usually considered straight.

Siphon: A pipe connecting two canals. A tube or pipe through which water flows over a high point by gravity.

Siphon tube: Relatively short, light-weight, curved tube used to convey water over ditch banks to irrigate furrows or borders.

Sink: Depression in the land surface, especially one having a central playa or saline lake with no outlet.

Sinkhole: A steep-sided depression formed when removal of subsurface embankment or foundation material causes overlying material to collapse into the resulting void.

Siphonage: A partial vacuum created by the flow of liquids in pipes.

Site: In archeology, any location of past human activity.

Skewed: On a horizontal angle, or in an oblique course or direction.

Skimmer gate: A gate at the spillway crest whose prime purpose is to control the release of debris and logs with a limited amount of water. It is usually a bottom hinged flap or bascule gate.

Skip: A non-digging bucket or tray that hoists material

Skiving: To dig in thin layers.

Slab: The deck or floor of a concrete bridge.

Slaking: The process of breaking up or sloughing when an indurated soil is immersed in water.

Sleet: Precipitation that consists of clear pellets of ice; sleet is formed when raindrops fall through a layer of cold air and freeze.

Slide: A small landslide.

Slide gate: A steel gate that upon opening or closing slides on its bearings in edge guide slots. A gate that can be opened or closed by sliding in supporting guides.

Slope: An inclined surface usually defined by the ratio of the horizontal distance to the vertical distance, i.e. 2:1 (2 horizontal units to 1 vertical unit). Change in elevation per unit of horizontal distance. Side of a hill or a mountain. The inclined face of a cut, canal, or embankment. Inclination from the horizontal. Expressed in percent when the slope is gentle; in this case also termed gradient. Degree of deviation of a surface from the horizontal, usually expressed in percent or degrees. Sometimes referred to as batter when measured from vertical.

Slope protection: The protection of an embankment slope against wave action or erosion.

Slough: Movement of a soil mass downward along a slope because of a slope angle too great to support the soil, wetness reducing internal friction among particles, or seismic activity. It is also called a slope failure, usually a rather shallow failure. A wet place of deep mud or mire, or a temporary or permanent lake; ordinarily found on or at the edge of the flood plain or a river. Also refers to a creek or sluggish body of water in a bottomland.

Slug: Large initial amount. A unit of mass which will undergo an acceleration of 1 foot per second squared when a force of 1 pound is applied to it.

Sluice: An opening for releasing water from below the static head elevation.

Sluice gate: A gate that can be opened or closed by sliding in supporting guides.

Sluiceway: An opening in a diversion dam used to discharge heavy floating debris safely past the dam.

Sluicing: A method of compacting soil where the soil is washed into placed with a high velocity stream of water.

Slurry: Watery mixture of insoluble matter which is pumped beneath a dam to form an impervious barrier. Cement grout.

Slurry trench: A narrow excavation whose sides are supported by a mud slurry filling the excavation. Sometimes used incorrectly to describe the cutoff itself.

Smolts: Adolescent salmon 3 to 7 inches long.

Snow: Precipitation that consists of frozen flakes formed when water vapor accumulates on ice crystals, going directly to the ice phase.

Social value (psychological value): Concept that the existence of wilderness provides a condition that could allow an individual to achieve control over stressful conditions, thus contributing to the psychological health of many off-site users.

Sodic soil: Contains sufficient exchangeable sodium to interfere with the growth of most crop plants. The sodium-adsorption ratio of the saturation extract is 15 or more.

Sodicity: The exchangeable-sodium content of the soil. High sodium content.

Soil: Sediments or other unconsolidated accumulations of solid particles produced by the physical and chemical disintegration of rocks, and which may or may not contain organic matter. Soil components may consist of clay, silt, sand, or gravel. The loose surface material of the earth's crust.

Soil cement: A mixture of water, cement, and natural soil, usually processed in a tumble and mixed to a specific consistency, then placed in lifts and rolled to compact to provide slope protection. A mixture of Portland cement and pulverized soil placed in layers on the upstream face of a dam to provide slope protection. A tightly compacted mixture of pulverized soil, Portland cement, and water that, as the cement hydrates, forms a hard, durable, low-cost paving material.

Soil-cement bedding: A mixture of soil, portland cement, and water placed for pipe bedding.

Soil classification: Systematic arrangement of soils into classes of one or more categories or levels of classification for a specific objective. Broad groupings are made on the basis of general characteristics and subdivisions are made on the basis of more detailed differences in specific properties. See Unified Soil Classification System.

Soil conservation: Protection of soil against physical loss by erosion and chemical deterioration by the application of management and land-use methods that safeguard the soil against all natural and human-induced factors.

Soil moisture: Water stored in soils.

Soil subsidence: The lowering of the normal level of the ground, usually due to overpumping of water from wells.

Solid head buttress dam: A buttress dam in which the upstream end of each buttress is enlarged to span the gap between buttresses. The terms "round head," "diamond head," "tee head" refer to the shape of the upstream enlargement. See massive head buttress dam.

Spacing: In blasting, the distance between holes in a row. Apparent spacing is the spacing as outlined by the delay pattern.

Spalling (spall): The loss of surface concrete usually caused by impact, abrasion, or compression. To break off from a surface in sheets or pieces.

Spatial concentration: The dry weight of sediment per unit volume of water-sediment mixture in place, or the ratio of the dry weight of sediment to the total weight of water-sediment mixture in a sample or a unit volume of the mixture.

Spawn: To lay eggs, refers mostly to fish.

Spawning beds: Places in which eggs of aquatic animals lodge or are placed during or after fertilization.

Special examination: A field review performed on a high- or significant-hazard dam to address an identified visible dam safety deficiency or to investigate significant changes in operating or loading conditions. Participation in these reviews may be by either the Denver, regional, and/or area offices depending on the nature of the concern.

Special features: Area containing ecological, geological, or other features of scientific, educational, scenic, or historical value.

Special populations: Those individuals or groups that may be institutionalized and have needs that require special consideration in emergencies.

Species:Basic category of biological classification intended to designate a single kind of animal or plant. See candidate species, endangered species, exotic species, extirpated species, obligate riparian species, sensitive species, and threatened species.

Specific gravity: The ratio of the mass of a body to an equal volume of water.

Specific weight: The weight per unit volume.

Spectrum intensity: The integral of the pseudo-velocity response spectrum taken over the range of significant structural vibration periods of the structure being analyzed.

Spigot: The plain end of a cast-iron pipe. The spigot is inserted into the bell end of the next pipe to make a water tight joint.

Spile (forepole): A plank driven ahead of a tunnel face for roof support.

Spills: Water releases that cannot be put to use for project purposes (includes floodflows).

Spillway: A structure that passes normal and/or flood flows in a manner that protects the structural integrity of the dam. Overflow channel of a dam or impoundment structure. A structure over or through which flow is discharged from a reservoir. If the rate of flow is controlled by mechanical means such as gates, it is considered a controlled spillway. If the geometry of the spillway is the only control, it is considered an uncontrolled spillway. Any passageway, channel, or structure designed to discharge surplus water from a reservoir. See auxiliary spillway, emergency spillway, service spillway, morning glory spillway, shaft spillway, and fuse plug spillway.

Spillway channel: An open channel or conduit conveying water from the spillway inlet downstream.

Spillway chute: A steeply sloping spillway channel that conveys discharges at super-critical velocities.

Spillway crest: The lowest level at which water can flow over or through a spillway.

Spinning reserves: Available capacity of generating facilities synchronized to the interconnected electric system where it can be called upon for immediate use in response to system problems or sudden load changes.

Splitter wall: A wall or pier parallel to the direction of flow in a channel that separates flows released from different sources as a means of energy dissipation.

Spoil: Dirt or rock which has been removed from its original location. Excavated material.

Spring: Ground water seeping or flowing out of the earth where the water table intersects the ground surface.

Springline: An imaginary horizontal reference line located at midheight, or halfway point, of a circular conduit, pipe, tunnel, or the point at which the side walls are vertical on a horseshoe-shaped conduit. Also the maximum horizontal dimension or diameter of a circular conduit, pipe, or tunnel. The meeting of the roof arch and the sides of a tunnel. The guideline for laying a course of bricks.

Sprinkler irrigation: A method of irrigation in which the water is sprayed, or sprinkled, through the air to the ground surface.

Stability: Tendency of systems, especially ecosystems, to persist, relatively unchanged, through time; also, persistence of a component of a system.

Staff gauge: A graduated scale on a plank or metal plate used to indicate the height of the water in a canal.

Stage: Same as elevation or depth of water. The height of a water surface above an established datum. See water surface elevation and gage height.

Standby reserves: Unused capacity in an electric system in machines that are not in operation but that are available for immediate use if required.

Standing Operating Procedures (SOP): A comprehensive single-source document covering all aspects of dam and reservoir operation and maintenance and emergency procedures. Its purpose is to ensure adherence to approved operating procedures.

Standpipe: Pipe or tank connected to a closed conduit and extending to or above the hydraulic grade line of the conduit to afford relief from surges of pressure in pipelines. A tank used for storage of water in distribution systems.

State Emergency Response Commission (SERC): Appointed by the governor of each state for the designation of emergency planning districts, appoint LEPC's, supervise and coordinate their activities, and review local emergency response plans as provided by SARA Title III.

Static head: The difference in elevation between the pumping source and the point of delivery. The vertical distance between two points in a fluid.

Station: Any one of a series of stakes or points indicating distance from a point of beginning or reference.

Station use: Energy used in a generating plant as necessary in production of electricity. Includes energy consumed for plant light, power, and auxiliaries regardless of whether such energy is produced at plant or comes from another source.

Steady flow: Flow in an open channel is said to be steady if the depth of flow does not change over a given time interval. No change occurs with repect to time.

Stemming: Crushed stone, soil, sand, or drill cuttings used to plug the unloaded portion of a drill hole.

Steam: Water vapor that rises from boiling water.

Stilling basin: Concrete portion downstream from conduit, tunnel, or control structure. A pool, usually lined with reinforced concrete, located below a spillway, gate, or valve into which the discharge dissipates energy to avoid downstream channel degradation. See terminal structure.

Stilling pool: A pool located below a spillway, gate, or valve into which the discharge dissipates energy to avoid downstream channel degradation. An unlined stilling basin usually constructed in natural ground or rock.

Stockpile: A storage pile of materials.

Stone: A concretion of earthy or mineral matter; rock.

Stoney gate: The fundamental difference between Stoney and fixed-wheel gates is that a moving train of rollers is substituted for the fixed wheels.

Stoplogs: Large logs, planks, cut timbers, steel or concrete beams placed on top of each other with their ends held in guides between walls or piers to close an opening in a dam, conduit, spillway, etc., to the passage of water; the logs are usually handled one at a time. Used to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate.

Strain: The change in length per unit of length in a given direction.

Stratification: Thermal layering of water in lakes and streams. Lakes usually have three zones of varying temperature, the epilimnion, the metalimnion, and the hypolimnion. The formation of separate layers (of temperature, plant, or animal life) in a lake or reservoir. See thermal stratification.

Stratified reservoir: A reservoir with several thermal layers of water.

Stratigraphy: Geology that deals with the origin, composition, distribution, and succession of strata (layers of rock).

Stream: Natural water course containing water at least part of the year. The type of runoff where water flows in a channel. See ephemeral stream, gaining stream, incised stream, intermittent stream, losing stream, or perennial stream.

Streambed at the dam axis: The elevation of the lowest point in the streambed at the axis or centerline crest of the dam prior to construction. This elevation defines the hydraulic height and normally defines the zero for the area-capacity tables.

Stream capacity: Total volume of water that a stream can carry within the normal high water channel.

Streamflow: Discharge that occurs in a natural channel.

Stream line: An imaginary line within the flow which is everywhere tangent to the velocity vector.

Stress: The force per unit area.

Strike: The direction taken by a bedding or fault plane as it intersects the horizontal. To be aligned or to trend in a direction at right angles to the direction of the dip.

Strip cropping: A crop production system that involves planting alternating strips of row crops and close-growing forage crops; the forage strips intercept and slow runoff from the less protected row crop strips.

Stripping: Removal of a surface layer or deposit for the purpose of excavating other material beneath it.

Structural Height: Distance between the lowest point in the excavated foundation (excluding narrow fault zones) and the top of dam. The structural height of a concrete dam is the vertical distance between the top of the dam and lowest point of the excavated foundation area, excluding narrow fault zones. The structural height of an embankment dam is the vertical distance between the top of the embankment and the lowest point in the excavated foundation area, including the main cutoff trench, if any, but excluding small trenches or narrow backfilled areas. The top elevation does not include the camber, crown, or roadway surfacing. See hydraulic height.

Strut: An inside brace.

Stud: A bolt having one end firmly anchored.

Subdrilling: Overdrilling or drilling below final grade. Sometimes necessary to assure that final grade is obtained when blasting.

Subgrade: Soil prepared and compacted to support a structure or pavement system.

Subirrigation: Applying irrigation water below the ground surface either by raising the water table within or near the root zone, or by use of a buried perforated or porous pipe system which discharge directly into the root zone.

Subsidence: Sinking of the land surface due to a number of factors, including ground water extraction.

Substation capacity: The substation capacities are given in kVA (kilovolt-amperes). To determine the load in kilowatts, which could be served from the transformers, the kilovolt-ampere rating should be multiplied by the load power factor.

Substrate: Surface on which a plant or animal grows or is attached. The base on which an organism lives; a substance acted upon.

Subsurface irrigation system: Irrigation by means of underground porous tile or its equivalent.

Succession: Directional, orderly process of community change in which the community modifies the physical environment to eventually establish an ecosystem which is as stable as possible at the site in question.

Sulfate attack: Damage to concrete caused by the effects of a chemical reaction between sulfates in soils or ground water and hydrated lime and hydrated calcium aluminate in cement paste. The attack results in considerable expansion and disruption of paste.

Sump: A pit or pool for draining, collecting, or storing water.

Sump pump: A pump used for removing collected water from a sump.

Supervisory control: A system used to monitor conditions and operate mechanical features associated with a facility from a location other than at the site.

Supplemental irrigation service land: Irrigable land now receiving, or to receive, an additional or reregulated supply of water through facilities constructed by or to be constructed by the Bureau of Reclamation. This water together with that obtained from nonproject sources, generally will constitute an adequate supply.

Suppressed weir: A rectangular weir that has only the crest far removed from the channel bottom, the sides are coincident with the sides of the approach channel, so no lateral contraction of water passing through the weir is possible.

Surcharge: To fill or load to excess. Any storage above the full pool.

Surcharge capacity (surcharge storage): The reservoir capacity provided for use in passing the inflow design flood through the reservoir. It is the reservoir capacity between the maximum water surface elevation and the highest of the following elevations: top of exclusive flood control capacity, top of joint use capacity, or top of active conservation capacity. Temporary storage.

Surface soil: Upper part of the soil ordinarily moved in tillage, or its equivalent in uncultivated soils, about 10 to 20 cm in thickness.

Surface vibration: A method of compacting soil using a vibrating plate or vibrating smooth drum roller used on the surface of soil placed.

Surface water: Water on the surface of the earth. An open body of water, such as a river, stream or lake.

Surge: A rapid increase in the depth of flow.

Surge irrigation: A surface irrigation technique wherein flow is applied to furrows (or less commonly, borders) intermittently during a single irrigation set.

Suspended: The state of floating in water rather than being dissolved in it.

Suspended load (suspended sediment): Sediment that is supported by the upward components of turbulence in a stream and that stays in suspension for an appreciable length of time.

Suspension: A method of sediment transport in which air or water turbulence supports the weight of the sediment particles, thereby keeping them from settling out or being deposited.

Swale: A low place in a tract of land. A wide, shallow ditch, usually grassed or paved.

Swell: Increase of bulk in soil or rock when excavated.

Syncline: A fold in rocks in which the strata dip inward from both sides toward the axis.

Tagout: The placement of a tagout device on an energy isolating device, in accordance with an established procedure, to indicate that the energy isolating device and the equipment being controlled may not be operated until the tagout device is removed. See lockout and clearance.

Tagout device: A prominent warning device, such as a tag and a means of attachment, which can be securely fastened to an energy isolating device in accordance with an established procedure, to indicate that the energy isolating device and the equipment being controlled may not be operated until the tagout device is removed.

Tailings: Second grade or waste material separated from pay material during screening or processing.

Tailings dam. See mine tailings dam.

Tailrace. See afterbay.

Tailwater: The water in the natural stream immediately downstream from a dam. The elevation of water varies with discharge from the reservoir. Applied irrigation water that runs off the lower end of a field. Tailwater is measured as the average depth of runoff water, expressed in inches or feet.

Tainter gate: A term used by the Corps of Engineers to describe radial gates (see gate).

Talus: Sloping accumulation of rock debris; also, rock fragments at the base of a cliff as the result of slides or falls. Rock fragments mixed with soil at the foot of a natural slope from which they have been separated.

Tamp: To pound or press soil to compact. To firmly compact earth during backfilling.

Tamper: A tool for compacting soil in spots not accessible to rollers.

Tamping: A method of compacting soil using the impact of a power or hand tamper on the surface of the soil placed.

Tamping roller: One or more steel drums, fitted with projection feet, used to densify soil. See sheepsfoot roller.

Tandem: A pair in which one part follows the other.

Tangent: A line that touches a circle and is perpendicular to its radius at the point of contact.

Taproot: A big root that grows downward from the base of a tree.

Tectonics: The science that deals with the structure of the earth's crust.

Tee: A pipe fitting that has two threaded openings in line, and a third at right angles to them.

Telescope: To slide one piece inside another.

Temporary irrigation service land: Irrigable land for which a water supply is delivered under temporary arrangements. The acreage may vary from year to year. When repayment and water service contracts are finalized upon such lands, they will be placed in either the Full or the Supplement Service category.

Temporary structure: Any structure that can be readily and completely dismantled and removed from the site between periods of actual use. The structure may or may not be authorized at the same site from season to season or from year to year.

Tensiometer: Instrument, consisting of a porous cup filled with water and connected to a manometer or vacuum gauge, used for measuring the soilwater matrix potential.

Terminal structure: Portion of spillway downstream from chute, tunnel or conduit, which generally dissipates or stills releases. Concrete portion of an outlet works downstream from a conduit, tunnel, or control structure. The structure dissipates or stills releases. See stilling basin.

Terrace: A ridge, a ridge and hollow, or a flat bench built along a ground contour. The surface form of a high sediment deposit having a relatively flat surface and steep slope facing the river. A broad channel, bench, or embankment constructed across the slope to intercept runoff and detain or channel it to protected outlets, thereby reducing erosion from agricultural areas.

Terrain: Ground surface.

Terrestrial: Living or growing on land. Land-based.

Territorial communities: Populations that function within a particular geographic area.

Test pit: Pit dug for geologic investigation or inspection and testing of earthwork placement.

Thalweg: Deepest part of a river channel in a cross section of a river profile. The path of deepest flow. Line connecting the deepest points along a riverbed. The lowest thread along the axial part of a valley. The middle or chief navigable channel of a waterway.

Thermal sand: Sand used to dissipate heat away from buried electrical cables.

Thermal stratification: The formation of layers of different temperatures in bodies of water. See stratification.

Thermocline: The middle layer of a lake, separating the upper, warmer portion (epilimnion) from the lower, colder portion (hypolimnion). The middle layer in a thermally stratified lake or reservoir. In this layer there is a rapid decrease in temperature with depth. See metalimnion.

Thermocouple: A heat-sensing device made of two conductors of different metals joined at their ends. An electric current is produced when there is a difference in temperature between the ends.

Thick arch dam: An arch dam with a base thickness to structural height ratio of 0.3 or greater (previously defined as 0.5 or greater).

Thickness or width of dam: The thickness or width of a dam as measured horizontally between the upstream and downstream faces and normal to the axis or centerline crest of the dam.

Thin arch dam: An arch dam with a base thickness to structural height ratio of 0.2 or less (previously defined as 0.3 or less).

Thixotrophy: The property of a material that enables it to stiffen in a relatively short time on standing but, upon agitation or manipulation, to change to a very soft consistency or to a fluid of high viscosity, the process being completely reversible.

Threatened: A legal classification for a species which is likely to become endangered within the foreseeable future.

Threatened species: Any species which has potential of becoming endangered in the near future.

Through cut: An excavation between parallel banks that begins and ends at original grade.

Thrust block (anchor block): A massive block of concrete built to withstand a thrust or pull. A mass of concrete or similar material appropriately placed around a pipe to prevent movement when the pipe is carrying water. Usually placed at bends and valve structures.

Tie lines: Transmission line connecting two or more power systems.

Tier 1: Reclamation public protection guideline dealing with loss of life (LOL) considerations. See annualized loss of life.

Tier 2: Reclamation public protection guideline dealing with public trust responsibilities based on the annual failure probability of the structure.

Tiering: The coverage of general matters in a broad National Environmental Policy Act document with subsequent narrowly focused documents; helps to eliminate repetitive discussions and allows the site-specific documents to focus on specific issues.

Tight: Soil or rock formations lacking veins of weakness.

Tile: Pipe made of baked clay.

Till: A deposit of sediment formed under a glacier, consisting of an unlayered mixture of clay, silt, sand, and gravel ranging widely in size and shape.

Tillage: Plowing, seedbed preparation, and cultivation practices. See conventional tillage.

Tilth: Soil condition in relation to lump or particle size.

Timber: Wood beams larger than 4 x 6.

Timbering: Wood bracing in a tunnel or excavation.

Timbering set: A tunnel support consisting of a roof beam or arch, and two posts.

Time of concentration: The time required for storm runoff to flow from the most remote point of a catchment or drainage area to the outlet or point under consideration. It is not a constant, but varies with depth of flow, grades, and condition of conduit and/or channel.

Toe: The point of intersection between the bottom of a slope or the upstream or downstream face of a dam and the natural ground, for example, the upstream or downstream toe of a dam or the downstream toe of a landslide or debris fan. For a concrete dam, see heel.

Toe drain(s): Open-jointed tile or perforated pipe located at the toe of the dam used in conjunction with horizontal drainage blankets to collect seepage from the embankment and foundation and conveys the seepage to a location downstream from the dam. A system of pipe and/or pervious material along the downstream toe of a dam used to collect seepage from the foundation and embankment and convey it to a free outlet.

Toe weight: Additional material placed at the toe of an embankment dam to increase its stability.

Top of active conservation capacity: The reservoir water surface elevation at the top of the capacity allocated to the storage of water for conservation purposes only. See reservoir.

Top of dam. See crest elevation.

Top of dead capacity: The lowest elevation in the reservoir from which water can be drawn by gravity.