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Backbar Channel — A channel formed behind a bar connected to the main channel but usually at a higher bed elevation than the man channel. Backbar channels may or may not contain flowing or standing water.

Bankfull Channel — The stream channel that is formed by the dominant discharge, also referred to as the active channel, which meanders across the floodplain as it forms pools, riffles, and point bars.

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 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.

Base Flow —
(1) The flow that a perennially flowing stream reduces to during the dry season. It is supported by groundwater seepage into the channel.
(2) 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.
(3) The volume of flow in a stream channel that is not derived from surface run-off. Base flow is characterized by los flow regime (frequency, magnitude, and duration daily, seasonally, and yearly), by minimum low flow events and in context of the size and complexity of the stream and its channel.

Base Level —
(1) The elevation to which a stream-channel profile has developed.
(2) The lowest level to which a land surface can be reduced by the action of running water.

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.

Basic Fixed Sites — Sites on streams at which streamflow is measured and samples are collected for temperature, salinity, suspended sediment, major ions and metals, nutrients, and organic carbon to assess the broad-scale spatial and temporal character and transport of inorganic constituents of stream water in relation to hydrologic conditions and environmental settings.

Bed Load —
(1) Sediment particles up to rock, which slide and roll along the bottom of the streambed.
(2) Material in movement along a stream bottom, or, if wind is the moving agent, along the surface.
(3) The sediment that is transported in a stream by rolling, sliding, or skipping along or very close to the bed. In USGS reports, bed load is considered to consist of particles in transit from the bed to an elevation equal to the top of the bed-load sample nozzle (usually within 0.25 feet of the streambed).

Bed Load Discharge — The quantity of sediment, typically measured in tons per day, that is moving as bed load, reported as dry weight, that passes a cross section in a given time.

Bed Sediment — The material at the bottom of a stream or other watercourse.

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

Benthic invertebrates — Insects, mollusks, crustaceans, worms, and other organisms without a backbone that live in, on, or near the bottom of lakes, streams, or oceans.

Benthic Organisms — Those organisms living at or near the bottom of a body of water. They include a number of types of organisms, such as bacteria, fungi, insect larvae and nymphs, snails, clams, and crayfish. They are useful as indicators of water quality.

Benthic Region — The bottom of a body of water, supporting the Benthos.

Benthos —
(1) All the plant and animals living on or closely associated with the bottom of a body of water.
(2) Organisms living within a streams’s substrate.

Best Available Demonstrated Technology (BADT) — The level of effluent limitation technology required by the 1972 Clean Water Act (CWA) to be used in setting new source performance standards for new industrial direct dischargers of water pollutants.

Best Available Technology Economically Achievable (BAT) — A national goal under the Water Pollution Control Act of 1972 (Public Law 92–500, commonly referred to as the Clean Water Act) which provides that industry shall use the best treatment technically and economically achievable for a category or class of point sources. Under this concept, pollution control will consider such factors as the age of the facilities and equipment involved, processes employed, engineering aspects of the control techniques, process changes, cost of the reductions, and environmental impacts other than water quality, including energy requirements.

Best Management Practice — Methods, measures, or practices that prevent or reduce water pollution. Best management practices may include treatment requirements, operating procedures, schedules of activities, prohibition of practices, maintenance procedures, or other management practices which control runoff, spillage, leaks, sludge or waste disposal, or drainage from various sites and operations.

Bifurcate — Dividing structure which splits the flow of water.

Bioaccumulation — The biological sequestering of a substance at a higher concentration than that at which it occurs in the surrounding environment or medium. Also, the process whereby a substance enters organisms through the gills, epithelial tissues, dietary, or other sources.

Bioavailability — The capacity of a chemical constituent to be taken up by living organisms either through physical contact or by ingestion.

Biodiversity — The variety of organisms found within a specified geographic region.

Biomonitoring — The use of living organisms to test the suitability of an effluent for discharge into receiving waters or to test the quality of such receiving waters downstream from the discharge.

Bioremediation — Simply, the use of biological techniques to clean up pollution. More specifically, the use of specialized, naturally-occurring micro-organisms with unique biological characteristics, appetites, and metabolisms as a form of waste cleanup. A critical underpinning of this process is the ability to economically generate a sufficient biomass of the appropriate microbes to accomplish in weeks or months what would normally take nature years to do. Typically, this is done either by applying a sufficient concentration of such microbes directly to the polluted area or by applying various concentrations of chemicals which, in turn, stimulate and foster the rapid growth of appropriate micro-organisms.