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A–Horizon — The uppermost zone in the Soil Profile, from which soluble Salts and Colloids are leached, and in which organic matter has accumulated. Generally this represents the most fertile soil layer and constitutes part of the Zone of Eluviation.

Abandoned Water Right — A water right which has not been put to Beneficial Use for generally five or more years, in which the owner of the water right states that the water right will not be used, or takes such actions that would prevent the water from being beneficially used.

Abandoned Well — A well which is no longer used or a well removed from service; a well whose use has been permanently discontinued or which is in a state of such disrepair that it cannot be used for its intended purpose. Generally, abandoned wells will be filled with concrete or cement grout to protect underground water from waste and contamination.

Abandonment (Water Right) —
(1) Generally refers to the intentional surrender of a water right by virtue of nonuse.
(2) Failure to put a water right to Beneficial Use for generally five or more years, in which the owner of the water right states that the water right will not be used, or takes such actions that would prevent the water from being beneficially used.

Abatement — Reducing the degree or intensity of, or eliminating, pollution, as a water pollution abatement program.

Abiota — Those non-living factors which are present in and affect the characteristics of a given ecosystem.

Ablation —
(1) The process by which ice and snow waste away as a result of melting and/or evaporation.
(2) The erosive processes by which a glacier is reduced.

Acre-Foot (AF)vA unit commonly used for measuring the volume of water; equal to the quantity of water required to cover one acre (43,560 square feet or 4,047 square meters) to a depth of 1 foot (0.30 meter) and equal to 43,560 cubic feet (1,234 cubic meters), or 325,851 gallons.

Adaptive Management — A process for implementing policy decisions as an ongoing activity that requires monitoring and adjustment. Adaptive management applies scientific principles and methods to improve resource management incrementally as managers learn from experience and as new scientific findings and social changes demand.

Adenosine Triphosphate (ATP) — An organic, phosphate-rich compound important in the transfer of energy in organisms. Its central role in living cells makes it an excellent indicator of the presence of living material in water. A measure of ATP therefore provides a sensitive and rapid estimate of Biomass. ATP is reported in micrograms per liter of the original water sample.

Adfluvial — Migrating between lakes and rivers or streams; typically used of fish species.

Advection — The process by which solutes are transported by the bulk of flowing fluid such as the flowing ground water.

Affluent (Stream) — A stream or river that flows into a larger one; a Tributary.

Agglomeration — (Water Quality) The grouping of small suspended particles into larger particles that are more easily removed through filtration, skimming, or settling. Also see Coagulation.

Aggradation —
(1) The raising of stream beds or flood plains by deposition of sediment eroded and transported from upstream.
(2) The build-up of sediments at the headwaters of a lake or reservoir or at a point where streamflow slows to the point that it will drop part or all of its sediment load.
(3) The building of a floodplain by sediment deposition; the filling of a depression or drainageway with sediment; the building of a fan by deposition of an alluvial mantle.
(4) Modification of the earth’s surface in the direction of uniformity of grade or slope, by Deposition, as in a river bed. Opposite of Degradation.

Aggrade — The raising of a stream-channel bed with time due to the Deposition of sediment that was eroded and transported from the upstream watershed or the channel.

Aggrading — The building up of a stream channel which is flowing too slowly to carry its sediment load.

Alachlor — A herbicide, marketed under the trade name Lasso, listed by the U.S. Environmental Protection Agency (EPA) as a “probable human carcinogen” and found frequently in streams and rivers, particularly following floods and periods of heavy rain. Alachlor is used extensively for weed control in corn, cotton, and soybean fields.

Alevin — Larval salmonid hatchling with yolk sac still attached. Alevin reside in the redd (nest) until the yolk sac is absorbed and they emerge from the gravel.

Algal Blooms — Rapid growth of algae on the surface of lakes, streams, or ponds; stimulated by nutrient enrichment. A heavy growth of algae in and on a body of water as a result of high phosphate concentration such as from farm fertilizers and detergents. It is associated with Eutrophication and results in a deterioration in water quality.

Allochthonous Material — Organic material that falls into a stream from the surrounding land. Compare to Autochthonous Material.

Allogenic — Exogenous, caused by external factors, such as a change in a habitat or environment caused by flooding. Contrast with Autogenic.

Allogenic Succession — Predictable changes in plant and animal communities in which changes are caused by events external to the community, for example, fire, drought, floods, etc.

Alluvial —
(1) Pertaining to processes or materials associated with transportation or deposition by running water.
(2) Pertaining to or composed of alluvium, or deposited by a stream or running water.
(3) An adjective referring to soil or earth material which has been deposited by running water, as in a riverbed, flood plain, or delta.

Alluvial Fan —
(1) A fan-shaped deposit of generally coarse material created where a stream flows out onto a gentle plain; a geomorphologic feature characterized by a cone or fan-shaped deposit of clay, silt, sand, gravel, and boulders that have been eroded from mountain slopes, transported by flood flows, and deposited on the valley floor.
(2) A geomorphologic feature characterized by a cone or fan-shaped deposit of boulders, gravel, and fine sediments that have been eroded from mountain slopes, transported by flood flows, and then deposited on the valley floors, and which is subject to flash flooding, high velocity flows, debris flows, erosion, sediment movement and deposition, and channel migration.
(3) (Montane) A semiconical, or fan-shaped constructional, major landform that is built of more-or-less stratified alluvium with or without debris flow deposits, that occurs on the upper margin of a piedmont slope, and that has its apex at a point source of alluvium debauching from a mountain valley into an inter-montane basis. Also, a generic term for like forms in various other landscapes.

Alluvial Fan Flooding — Flooding occurring on the surface of an Alluvial Fan or similar landform which originates at the apex and is characterized by high-velocity flows, active processes of erosion, sediment transport, deposition, and unpredictable flow paths.

Alluvion —
(1) The flow of water against a shore or bank. Inundation by water; flood.
(2) (Legal) The increasing of land area along a shore by deposited Alluvium or by the recession of water.

Amalgamation — The dissolving or blending of a metal (commonly gold and silver) in mercury to separate it from its parent material.

Anadromous — Ascending rivers from the sea, at certain seasons, for breeding. For example, salmon and shad are anadromous fish.

Anastomosing — The branching and rejoining of channels to form a netlike pattern.

Anthropogenic — Involving the impact of man on nature; induced, caused, or altered by the presence and activities of man, as in water and air pollution.

Aquatic guidelines — Specific levels of water quality which, if reached, may adversely affect aquatic life. These are nonenforceable guidelines issued by a governmental agency or other institution.

Aquatic-life criteria — Water-quality guidelines for protection of aquatic life. Often refers to U.S. Environmental Protection Agency water-quality criteria for protection of aquatic organisms. See also Water-quality guidelines and Water-quality standards.

Aquifer — A water-bearing layer of soil, sand, gravel, or rock that will yield usable quantities of water to a well.

Area of Critical Environmental Concern (ACEC) — An area on Public Lands where special management attention is required to protect and prevent irreparable damage to historic, cultural, or scenic values, fish and wildlife resources, or other natural systems or processes, or to protect people from natural hazards.

Artificial Substrate —
(1) A device placed in the water for a specified period of time that provides living spaces for a multiplicity of organisms; for example, glass slides, concrete blocks, multi-plate samplers, or rock baskets; used primarily to collect organisms in areas where the physical habitat is limiting or cannot be adequately sampled using conventional methods.
(2) A device which is purposely placed in a stream or lake for colonization of organisms. The artificial substrate simplifies the community structure by standardizing the substrate from which each sample is taken. Examples of artificial substrates are basket samplers (made of wire cages filled with clean streamside rocks) and multiplate samplers (made of hardboard) for benthic organism collection, and plexiglass strips for periphyton collection.

Assessment Report — A comprehensive record of historical, existing and projected water quality conditions of a particular watershed.

Assimilation — The ability of a body of water to purify itself of pollutants.

Assimilative Capacity — The ability of air, a natural body of water, or soil to effectively degrade and/or disperse chemical substances. If the rate of introduction of pollutants into the environment exceeds its assimilative capacity for these substances, then adverse effects may result to habitat and wildlife.

Attenuation —
(1) Generally, a term used to describe the slowing, modification, or diversion of the flow of water as with Detention and Retention.
(2) (Water Quality) The process of diminishing contaminant concentrations in ground water, due to filtration, biodegradation, dilution, sorption, volatilization, and other processes.

Autochthonous Material —
(1) Pertaining to substances, materials, or organisms originating within a particular waterway and remaining in that waterway.
(2) Organic material produced in the stream usually through primary production.

Avulsion —
(1) The sudden movement of soil from one property to another as a result of a flood or a shift in the course of a boundary stream. (2) A sudden cutting off or separation of land by a flood or by an abrupt change in the course of a stream, as by a stream breaking through a meander or by a sudden change in current, whereby the stream deserts its old path for a new one. (4) A sudden loss or gain of land as the result of action of water or a shift in a bed of a river which has been used as a boundary by property owners. If land is lost as a result of avulsion the riparian owner does not lose title to the land that has been lost; the boundary lines remain the same. This is not true when land is lost by erosion.