BACKFILL, or Backfilling — process of filling the notches carved in the earth from strip mining in order to restore the original slope. This is intended to reduce soil erosion and allow for the reestablishment of vegetation.

BACKFLOW — the backing up of water through a conduit or channel in the direction opposite to normal flow. A reverse flow condition created by a difference in water pressures that causes water to flow back into the distribution pipes of a drinking water supply from any source other than the intended one. Also referred to as Back Siphonage.

BACKGROUND — Value for a parameter that represents the conditions in a system prior to a given influence in space or time.

BACK PRESSURE — a pressure that can cause water to Backflow into the water supply when a user's waste water system is at a higher pressure than the public system.

BACK SIPHONAGE — A reverse flow condition created by a difference in water pressures that causes water to flow back into the distribution pipes of a drinking water supply from any source other than the intended one.

BACKWASH — A backward flow or water, also referred to as Backrush. (Water Quality) The reversal of flow through a rapid sand filter to wash clogging material out of the filtering medium and reduce conditions causing loss of head (pressure).

BACKWASHING — In a wastewater or water treatment facility, the flow of clean water in a direction opposite (upward) to the normal flow of raw water through rapid sand filters in order to clean them.

BACKWATER — (1) A small, generally shallow body of water attached to the main channel, with little or no current of its own. (2) Water backed up or retarded in its course as compared with its normal or natural condition of flow. In Stream Gauging, a rise in Stage produced by a temporary obstruction such as ice or weeds, or by the flooding of the stream below. The difference between the observed stage and that indicated by the Stage-Discharge Relation, is reported as backwater.

BACKWATER CURVE — The longitudinal profile of the water surface in an open channel where the water surface is raised above its normal level by a natural or artificial obstruction. The term is sometimes used in a generic sense to denote all water surface profiles, or profiles where the water is flowing at depths greater than critical.

BACKWATER EFFECT — The rise in surface elevation of flowing water upstream from and as a result of an obstruction to flow. In stream gaging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by the flooding of the stream below. The difference between the observed stage and that indicated by the stage-discharge relation is reported as backwater.

BACKWATER FLOODING — Flooding caused by a restriction or blocking of flow downstream. Examples include a narrowing of the channel, logjam, ice jam, high flow in a downstream confluence stream, or high tide blocking high river flows from entering estuaries.

BACKWATER POOLS — A pool type formed by an eddy along channel margins downstream from obstructions such as bars, rootwads, or boulders, or resulting from backflooding upstream from an obstructional blockage. Backwater pools are sometimes separated from the channel by sand or gravel bars.

BAG OF WATERS — The double-walled fluid-filled sac that encloses and protects the fetus in the womb and that breaks releasing its fluid during the birth process

BAIL — To remove water, as from the bottom of a boat or other vessel.

BAILER — An instrument such as a long pipe with a valve at the lower end used to extract a water sample from a groundwater well. Also used to remove slurry from the bottom or side of a well as it is being drilled.

BALANCED OPERATION — Operation of a canal system where the water supply exactly matches the total flow demand.

BALANCED GROUNDWATER SCENARIO (BGS) — A term referring to the development of a scenario exploring changes in cropping patterns such that long-term ground water withdrawals do not exceed long-term groundwater recharge rates.

BALL VALVE — A valve regulated by the position of a free-floating ball that moves in response to fluid or mechanical pressure.

BANK, and BANKS — The slope of land adjoining a body of water, especially adjoining a river, lake, or a channel. With respect to flowing waters, banks are either right or left as viewed facing in the direction of the flow. As Banks, a large elevated area of a sea floor.

BANK AND CHANNEL STABILIZATION — Implementation of structural features along a streambank to prevent or reduce bank erosion and channel degradation.

BANK STORAGE — The water absorbed into the banks of a stream, lake, or reservoir, when the stage rises above the water table in the bank formations, then returns to the channel as effluent seepage when the stage falls below the water table. Bank storage may be returned in whole or in part as seepage back to the water body when the level of the surface water returns to a lower level.

BARRAGE — An artificial obstruction, such as a dam or an irrigation channel, built in a watercourse to increase its depth or to divert its flow either for navigation or irrigation. Sometimes the purpose is to control peak flow for later release.

BASALT AQUIFERS — Aquifers found in basalt rock in areas of past volcanic activity, particularly in the Pacific Northwest region of the United States and in Hawaii.

BASE — (1) Any of various typically water-soluble and bitter tasting compounds that in solution have a pH greater than 7, are capable of reacting with an acid to form a salt, and are molecules or ions able to take up a proton from an acid or able to give up an unshared pair of electrons to an acid. (2) Chemicals that release hydroxide ions (OH-) in solution. Such solutions have a soapy feel, neutralize acids, and conduct electricity.

BASE FLOOD (100-YEAR FLOOD) — The flood having a 1 percent average probability of being equaled or exceeded in a given year at a designated location. It may occur in any year or even in successive years if the hydrologic conditions are conducive for flooding. Also see Hundred-Year Flood, X-Year Flood, and X-Year Flood, Y-Duration Rain.

BASE FLOOD ELEVATION — The height in relation to mean sea level (MSL) expected to be reached by the waters of the base flood at pertinent points in the floodplain of Riverine areas.

BASE FLOODPLAIN — The floodplain that would be inundated by a one percent chance flood (100-Year Flood).

BASE FLOW — The fair-weather or sustained flow of streams; that part of stream discharge not attributable to direct runoff from precipitation, snowmelt, or a spring. Discharge entering streams channels as effluent from the groundwater reservoir. Also referred to as Groundwater Flow.

BASE LEVEL — The lowest level to which a land surface can be reduced by the action of running water.

BASELINE — The condition that would prevail if no action were taken.

BASELINE (DATA) — A quantitative level or value from which other data and observations of a comparable nature are referenced. Information accumulated concerning the state of a system, process, or activity before the initiation of actions that may result in changes.

BASE PERIOD — A period of time specified for the selection of data for analysis. The base period should be sufficiently long to contain data representative of the averages and deviations from the averages that must be expected in other periods of similar and greater length. For example, the U.S. Weather Bureau computes values of average, heavy, and light monthly precipitation from data observed during the base period of 1931-1960. For ground-water studies, the base period should both begin and end at the conclusion of a dry trend so that the difference between the amount of water in transit in the soil at the ends of the base period is minimal.

BASE RUNOFF — Sustained or fair weather runoff. In most streams, base runoff is composed largely of ground-water effluent. The term base flow is often used in the same sense as base runoff. However, the distinction is the same as that between streamflow and runoff. When the concept in the terms base flow and base runoff is that of the natural flow in a stream, base runoff is the more appropriate term.

BASE WIDTH — (1) The time interval between the beginning and end of the direct runoff produced by a storm. (2) The time period covered by a Unit Hydrograph.

BASIC — Describing a solution, sediment, or other material that has a pH greater than 7.0. see Alkaline

BASIC HYDROLOGIC DATA — Includes inventories of features of land and water that vary only from place to place (e.g., topographic and geologic maps), and records of processes that vary with both place and time (e.g., records of precipitation, streamflow, ground-water, and quality-of-water analyses). Basic Hydrologic Information is a broader term that includes surveys of the water resources of particular areas and a study of their physical and related economic processes, interrelations and mechanisms.

BASIN — (1) A geographic area drained by a single major stream; consists of a drainage system comprised of streams and often natural or man-made lakes. Also referred to as Drainage Basin, Watershed, or Hydrographic Region. (2) A naturally or artificially enclosed harbor for small craft, such as a yacht basin.

BASIN FILL — Unconsolidated material such as sand, gravel, and silt eroded from surrounding mountains and deposited in a valley.

BASIN LAG — (1) The time from the centroid (centermost point in time based on total period rainfall) of rainfall to the hydrograph peak. (2) The time from the centroid of rainfall to the centroid of the Unit Hydrograph.

BASIN MANAGEMENT (of Water) — Also referred to as Water or Watershed Management, it is the analysis, protection, development, operation, or maintenance of the land, vegetation, and water resources of a drainage basin for the conservation of all its resources for the benefit of man. Basin management for water production is concerned with the quality, quantity, and timing of the water which is produced.

BASIN YIELDS — The amount of water which will flow from a drainage or catchment area in a given storm.

BATHYMETRIC MAP — A map showing the depth (bottom contours) of water in lakes, streams, or oceans. Can be used to calculate lake volume.

BATHYMETRY — (1) The measurement of the depth of large bodies of water (oceans, seas, ponds and lakes). (2) The measurement of water depth at various places in a body of water. Also the information derived from such measurements.

BED — (1) An underwater or intertidal area in which a particular organism is established in large numbers. (2) The bottom of a body of water, such as a stream. (Geology) A rock mass of large horizontal extent bounded, especially above, by physically different material (as in Bedrock).

BEHEADED STREAM — The lower section of a stream that has lost its upper portion through diversion or Stream Piracy.

BENCHMARK — Data used as a base for comparative purposes with comparable data.

BENEFICIAL USE (of Water) — (1) The amount of water necessary when reasonable intelligence and diligence are used for a stated purpose. Most states recognize the following uses as beneficial:
[1] domestic and municipal uses;
[2] industrial uses;
[3] irrigation;
[4] mining;
[5] hydroelectric power;
[6] navigation;
[7] recreation;
[8] stock raising;
[9] public parks;
[10] wildlife and game preserves.

(2) The cardinal principle of the (Prior) Appropriation Doctrine. A use of water that is, in general, productive of public benefit, and which promotes the peace, health, safety and welfare of the people of the State. A certificated water right is obtained by putting water to a beneficial use. The right may be lost if beneficial use is discontinued. A beneficial use of water is a use which is of benefit to the appropriator and to society as well. The term encompasses considerations of social and economic value and efficiency of use. In the past, most reasonably efficient uses of water for economic purposes have been considered beneficial. Usually, challenges have only been raised to wasteful use or use for some non-economic purpose, such as preserving in-stream values. Recent statutes in some states have expressly made the use of water for recreation, fish and wildlife purposes, or preservation of the environment a beneficial use

BENEFIT-COST RATIO — The relationship of the economic benefits of an action to its total costs.

BENTHIC DEPOSITS — Bottom accumulations which may contain bottom-dwelling organisms and/or contaminants in a lake, harbor, or stream bed.

BERNOULLI EFFECT — The phenomenon of internal pressure reduction with increased stream velocity in a fluid. Named after Daniel Bernoulli.

BERNOULLI'S EQUATION — Under conditions of steady flow of water, the sum of the velocity head, the pressure head, and the head due to elevation at any given point is equal to the sum of these heads at any other point plus or minus the head losses between the points due to friction or other causes.

BERM — (1) A narrow ledge or path as at the top or bottom of a slope, stream bank, or along a beach. (2) (Dam) A horizontal step or bench in the upstream or downstream face of an Embankment Dam.

BILLOW — A large wave or swell of water.

BIODIVERSITY — Refers to the variety and variability of life, including the complex relationships among microorganisms, insects, animals, and plants that decompose waste, cycle nutrients, and create the air that we breathe. Diversity can be defined as the number of different items and their relative frequencies. For biological diversity, these items are organized at many levels, ranging from complete Ecosystems to the biochemical structures that are the molecular basis of heredity. Thus, the term encompasses different ecosystems, species, and genes. It is generally accepted that human survival is dependent upon the conservation and preservation of this diversity of life forms. Typically five levels of biodiversity are recognized:
[1] Genes — Genetic diversity encompasses the variety of genetically coded characteristics of plant and animal populations;
[2] Populations — Groups of individuals of a species that interbreed or interact socially in an area;
[3] Species — The level at which most organisms are recognizable as distinct from all others;
[4] Natural Communities — Groups of species that typically occur in recognizable units, such as redwood forests, coastal sage scrub, or oak woodlands. A natural community includes all the vegetation and animal life, and their interactions within that community; and
[5] Ecosystems — A collection of natural communities. An ecosystem can be as small as a rotting log or a puddle of water, but current management efforts typically focus on larger landscape units, such as a mountain range, a river basin, or a watershed.

BIOINDICATOR — A living organism that denotes the presence of a specific environmental condition. For example, the presence of coliform bacteria identifies water that is contaminated with human fecal material.

BIOLOGICAL PROCESSES — Processes characteristic of, ore resulting from, the activities of living organisms.

BIOLOGICAL TREATMENT — A treatment technology that uses bacteria to consume organic wastes.

BIOLOGICAL WASTEWATER TREATMENT — The use of bacteria to degrade and decompose organic materials in wastewater.

BLANKET (of a Dam) — A portion of the physical structure of a dam designed to affect the dams hydrologic characteristics, particularly its seepage and strength characteristics. Types of dam blankets include:
[1] Drainage Blanket — A drainage layer placed directly over the dam's foundation material;
[2] Grout Blanket — The injection of grout to consolidate a layer of the foundation, resulting in greater impermeability and/or strength; and
[3] Upstream Blanket — An impervious layer placed on the reservoir floor upstream of a dam; in the case of an Embankment Dam, the blanket may be connected to the impermeable element in the dam itself.

BLENDING — The mixing or combination of one water source with another, typically a finished source of water with raw water to reuse water while still satisfying water quality standards, for example, mixing of product water from a desalting plant with conventional water to obtain a desired dissolved solids content, or mixing brine effluents with sewage treatment plant effluents in order to reduce evaporation pond size.

BLINDS — Water samples containing a chemical of known concentration given a fictitious company name and slipped into the sample flow of the lab to test the impartiality of the lab staff.

BLOWDOWN — The water drawn from boiler systems and cold water basins of cooling towers to prevent the buildup of solids.

BORDER IRRIGATION — A surface method of irrigation by flooding between two confining border levees or dikes. Typically, these borders vary from 100 to 200 feet wide by 1,000 to 3,960 feet long.

BOTTOM — (1) The deepest or lowest part, as the bottom of a well. (2) The solid surface under a body of water. (3) Often Bottoms: Low-lying alluvial land adjacent to a river, also referred to as bottomland. (4) (Nautical) The part of a ship's hull below the water line.

BOTTOMLAND, also Bottom Land (Soils) — A general term describing generally rich, loamy or fine-textured and poorly drained soils, overlying a shallow water table or possibly adjacent to a stream, lake or other body of water, that exhibits relatively good water holding capacity and slow to moderate infiltration of irrigation water; often associated with a river's floodplain.

BOTTOM LAND HARDWOODS — Forested freshwater Wetlands adjacent to rivers in the southeastern United States, especially valuable for wildlife breeding, nesting, and habitat.

BOUNDARY CONDITIONS — Flow conditions imposed at the ends of a pipeline or canal reach by various physical structures, which must be described mathematically to solve the general equation of flow for hydraulic transient computer models.

BOUNDARY LAYER — The layer of reduced velocity in fluids, such as air and water, that is immediately adjacent to the surface of a solid past which the fluid is flowing.

BOUND WATER — Water molecules that are held tightly to soil or other solids. This water is not easily removed by normal drying and is not available for other purposes such as plant growth.

BRACKISH WATER — Generally, water containing dissolved minerals in amounts that exceed normally acceptable standards for municipal, domestic, and irrigation uses. Considerably less saline than sea water. Also, Marine and Estuarine waters with Mixohaline salinity (0.5 to 30 due to ocean salts). Water containing between 1,000-4,000 parts per million (PPM) Total Dissolved Solids (TDS). The term brackish water is frequently interchangeable with Saline Water. The term should not be applied to inland waters.

BRAIDED STREAM — A complex tangle of converging and diverging stream channels (Anabranches) separated by sand bars or islands. Characteristic of flood plains where the amount of debris is large in relation to the discharge.

BRANCH — (1) A tributary of a river or other body of water. (2) A divergent section of a river, especially near the mouth.

BRIDGE — An over the lake, stream or river structure built so that people can get from one side to the other.

BRIM — The upper surface of a body of water.

BRINE — (1) Water saturated with or containing large amounts of a salt, especially of sodium chloride. According to U.S. Geologic Survey (USGS) classification, water classified as brine contains more than 35,000 ppm (parts per million) total dissolved solids (TDS) of salt; (2a) The water of a sea or an ocean; (2b) A large body of salt water. (3) The wastewater resulting from desalting. It is higher in dissolved solid content than feedwater or product water.

BRINE DISPOSAL — Removing water that contains high concentrations of salt.

BROOK — A natural stream of water, smaller than a river or creek; especially a small stream or rivulet which breaks directly out of the ground, as from a spring or seep; also, a stream or torrent of similar size, produced by copious rainfall, melting snow and ice, etc.; a primary stream not formed by tributaries, though often fed below its source, as by rills or runlets; one of the smallest branches or ultimate ramifications of a drainage system.

BURIED DRAIN — A covered drain usually made of clay, concrete, or plastic pipe installed beneath the ground surface at a planned grade and depth for conveyance of excess groundwater.

BUTTRESS DAM — A dam consisting of a watertight upstream face supported at intervals on the downstream side by a series of buttresses.

BYPASS SYSTEM — A structure in a dam that provides a route for fish to move through or around the dam without going through the turbines.