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.  Compare to Forfeited Water Right.
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.  Also see Abandoned Water Right.  Compare to Forfeiture (Water Right).
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.
Abrasion Removal of stream-bank soil as a result of sediment-laden water, ice, or debris rubbing against the bank.
Abscissa (Symbol X) (Mathematics) The coordinate representing the position of a point along a line perpendicular to the y-axis (Ordinate) in a Plane Cartesian Coordinate System.
Abscission The dropping of leaves from a plant.  Premature abscission in certain plant species frequently results from excessive exposure to certain air contaminants.
Absolute Humidity The actual weight of water vapor contained in a unit volume of the atmosphere, usually expressed in grams of water per kilogram of air.  Compare to Relative Humidity.
Absolute Temperature (T) A temperature expressed on the thermodynamic scale, measured from Absolute Zero, or 0?Kelvin (K), also equivalent to -273.15?C or -459.67?F.
Absolute Zero The zero value of thermodynamic temperature, or 0?Kelvin (?K), also equivalent to -273.15?Celsius (?C) on the Centigrade Temperature Scale or -459.67?Fahrenheit (?F) on the Fahrenheit Temperature Scale.
Absorber A material capable of taking in a substance, such as oil, as a sponge takes up water.
Absorption (1) The entrance of water into the soil or rocks by all natural processes, including the infiltration of precipitation or snowmelt, gravity flow of streams into the valley alluvium into sinkholes or other large openings, and the movement of atmospheric moisture.  (2) The uptake of water or dissolved chemicals by a cell or an organism (as tree roots absorb dissolved nutrients in soil).  (3) More generally, the process by which substances in gaseous, liquid, or solid form dissolve or mix with other substances.  Not to be confused with Adsorption.
Absorption Loss The loss of water by Infiltration or Seepage into the soil during the process of priming, i.e., during the initial irrigation of a field; generally expressed as flow volume per unit of time.
Absorption Tower (Air Quality) An air pollution control device in which contaminated air is passed through a tower containing substances (packing) possessing large surface area.  Water is passed over the packing material in a countercurrent fashion, i.e., in a direction opposite to the passage of the air, and the air contaminants are then absorbed into the liquid.  Also referred to as Packed Tower, Spray Tower, or Tray Tower.
Abutment (of a Dam) The part of a valley side wall against which a dam is constructed.  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.  Right and left abutments are designated as one looks downstream.
Abutment Seepage Reservoir water that moves through seams or pores in the dam's natural Abutment material and exists as seepage.
Abyssal Of or relating to the bottom waters of the ocean depth.
Abyssal Zone The bottom of a deep ocean.  Also see Bathyal Zone and Euphotic Zone.
Acclimatization The physiological adjustment or adaptation by an organism to new physical and/or environmental conditions.  With respect to water, it is frequently used in reference to the ability of a species to tolerate changes in water temperature, degradation of water quality, or increased levels of salinity.
Accretion The slow addition to land by deposition of water-borne sediment.  An increase in land along the shores of a body of water, as by Alluvial deposit.
Acequia (Southwestern U.S.) (1) An irrigation canal; an irrigation ditch or channel, a term commonly used in northern New Mexico.  (2) A Spanish word used in the Southwestern United States in referring to a community irrigation ditch or canal.  (3) Community-run irrigation ditches and/or the community-run organizations that manage them.  These systems of water management are rooted in ancient Spanish custom and many still operate in northern New Mexico.
Acid (1) Chemicals that release hydrogen ions (H+) in solution and produce hydronium ions (H3O+).  Such solutions have a sour taste, neutralize bases, and conduct electricity.  (2) Term applied to water with a pH of less than 7.0 on a pH scale of 0 to 14.
Acid Aerosol Airborne particles composed of sulfates (SOX), sulfuric acid (H2SO4), nitrates (NOX), and/or nitric acid (HNO3).  Dry particle diameters are typically less than 1-2 microns.  Also see Acid Deposition and Acid Fog.
Acid Deposition The introduction of acidic material to the ground or to surface waters.  Involves a complex chemical and atmospheric phenomenon that occurs when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and then become deposited on the land or surface waters in either wet or dry forms.  Wet Deposition (commonly referred to as Acid Rain or Acid Fog) results from precipitation as rain, snow, or fog.  Dry Deposition results from particle fallout or acidic gases.
Acid Fog Airborne water droplets containing sulfuric acid and/or nitric acid.  Typical diameters are 3-30 microns.  Also see Acid Deposition and Acid Aerosol.
Acid-Forming Material Material containing sulfide minerals or other materials, which if exposed to air, water, or weathering processes will form sulfuric acid that may create Acid Mine Drainage.
Acid Mine Drainage (AMD) Acidic water that flows into streams from abandoned mines or piles of mining waste or tailings.  The acid arises from the oxidation of iron sulfide compounds in the mines by air, dissolved oxygen in the water, and chemoautotrophs, which are bacteria that can use the iron sulfide as an energy source.  Iron sulfide oxidation products include sulfuric acid, the presence of which has reduced or eliminated aquatic life in many streams in mining regions.  Also see Open-Pit Mining and Yellowboy.  Also referred to as Acid Mine Waste.
Acid Neutralizing Capacity (ANC) (1) A measure of the ability of water or soil to resist changes in pH.  (2) The equivalent sum of all bases or base-producing materials, solutes plus particulates, in an aqueous system that can be titrated with acid to an equivalence point.  The term designates titration of an "unfiltered" sample (formerly reported as alkalinity).
Acid Precipitation Atmospheric precipitation that is composed of the hydrolyzed by-products from oxidized halogen, nitrogen, and sulfur substances.  Also see Acid Rain.
Acid Rain Rainfall with a pH of less than 7.0.  One of the principle sources is the combining of rain (H2O) and sulfur dioxide (SO2), nitrous oxides (NOx), and carbon dioxide (CO2) emissions which are byproducts of the combustion of fossil fuels.  These oxides react with the water to form sulfuric (H2SO4), nitric (HNO3), and carbonic acids (H2CO3).  Long-term deposition of these acids is linked to adverse effects on aquatic organisms and plant life in areas with poor neutralizing (buffering) capacity.  Also see Acid Deposition.
Acid Soil (Alkaline Soil, Neutral Soil) A description of one aspect of a soil's chemical composition.  Many plants will grow best within a range of pH rating from slightly acid to slightly alkaline.  A pH rating of 7 means that the soil is neutral; a pH below 7 indicates acidity; a pH above 7 indicates alkalinity.
Acidic The condition of water or soil that contains a sufficient amount of acid substances to lower the pH below 7.0.
Acidification Raising the acidity (lowering the pH) of a fluid by adding an acid.
Acidity A measure of how acid a solution may be.  A solution with a pH of less than 7.0 is considered acidic.  Solutions with a pH of less than 4.5 contain mineral acidity (due to strong inorganic acids), while a solution having a pH greater than 8.3 contains no acidity.
Acre A measure of area equal to 43,560 square feet (4,046.87 square meters).  One square mile equals 640 acres, and is also referred to as a Section.
Acre-Feet (AF) A unit commonly used for measuring the volume of water.  See Acre-Foot.
Acre-Foot (AF)  A 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.
Acre-Inch The volume of water or solids that will cover one acre to a depth of one inch, equivalent to 3,630 cubic feet or 102.7 cubic meters.
Activated Carbon A material produced by heating coal or wood in such a manner as to yield a porous structure, creating a very large internal surface area.  Activated carbon is available in both powdered and granular forms, and is widely used to adsorb organic compounds from water and wastewater.  It provides a means of removing tastes and odors from drinking water.  Also see Granular Activated Carbon (GAC).  Also referred to as Activated Charcoal.
Activated Carbon Adsorption The process of pollutants moving out of water and attaching on to Activated Carbon.
Activated Sludge The Floc produced in raw or settled wastewater due to the growth of bacteria and other organisms in the presence of Dissolved Oxygen.  It is the product that results when primary effluent is mixed with bacteria-laden sludge and then agitated and aerated to promote biological treatment, speeding the breakdown of organic matter in raw sewage undergoing secondary waste treatment.
Activated Sludge Process A method of Secondary Wastewater Treatment in which the waste is treated by microorganisms in a well-aerated tank to degrade the organic material.  A sedimentation tank is then used to remove the resultant sludge.
Active Conservation Storage Storage of water for later release for purposes, such as municipal and industrial (M&I) uses, hydropower, or irrigation.
Active Fault A fault that has undergone movement in recent geologic time (the last 10,000 years) and may be subject to future movement.  Also see Fault.
Active Solar Water Heater A water heating system in which heat from the sun is absorbed by collectors and transferred by pumps to a storage unit.  The heated fluid in the storage unit conveys its heat to the domestic hot water system of the house through a heat exchanger.
Active Storage Capacity (1) The total amount of usable reservoir capacity available for seasonal or cyclic water storage.  It is gross reservoir capacity minus inactive storage capacity.  (2) More specifically, the volume of water in a reservoir below the maximum controllable level and above the minimum controllable level that can be released under gravity.  In general, it is the volume of water between the outlet works and the spillway crest.  In some instances, Minimum Pool operating constraints may prevent lowering the reservoir to the level of the outlet works, and the water below the minimum pool level is not considered to be in active storage.
Activity The effective concentration of a chemical based on thermodynamic considerations.  Activity and concentration have the same units and have the same value in very dilute solutions.
Acute Designates an exposure to a dangerous substance or chemical in sufficient dosage to precipitate a severe reaction.  Acute Exposure refers to such dosage levels received over a period of 24 hours or less.  Longer-term exposures are referred to as Chronic Exposure.
Adaptation Changes in an organism's structure or habits that allow it to adjust to its surroundings.
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.
Adequate-Size Farm A farm with resources and productivity sufficient to generate enough income to (a) provide an acceptable level of family living; (b) pay current operating expenses and interest on loans; and (c) allow for capital growth to keep pace with technological growth.
Adfluvial Migrating between lakes and rivers or streams; typically used of fish species.
Adhesion Molecular attraction that holds the surfaces of two substances in contact, such as water and rock particles.  Also, the attraction of water molecules to other materials as a result of hydrogen bonding.
Adiabatic Applies to a thermodynamic process during which no heat is added to or withdrawn from the body or system concerned.  In the atmosphere, adiabatic changes of temperature occur only in consequence of compression or expansion accompanying an increase or decrease of atmospheric pressure.  Thus, a descending body of air undergoes compression and adiabatic cooling.
Adiabatic Lapse Rate The theoretical rate at which the temperature of the air changes with altitude.  The temperature change is due to the pressure drop and gas expansion only, and no heat is considered to be exchanged with the surrounding air through convection or mixing.  The Dry Adiabatic Lapse Rate for air not saturated with water vapor is 0.98?C per 100 meters (5.4?F per 1,000 feet).  The Wet Adiabatic Lapse Rate for air saturated with water vapor is about 0.60?C per 100 meters (3.3?F per 1,000 feet).
Adiabatic Process A change involving no gain or loss of heat.
Adit A horizontal or nearly horizontal passage, driven from the surface, for the working or dewatering of a mine.  Also referred to as Drift, Shaft, or Portal.
Adjudicate To determine by judicial action.
Adjudication (1) Refers to a judicial process whereby water rights are determined or decreed by a court of law.  (2) A court proceeding to determine all rights to the use of water on a particular stream system or within a specific ground water basin.
Administered Groundwater Basin A groundwater basin (watershed, area, or sub-area) which, in the interest of public welfare, is monitored by an appropriate agency to insure adequate water resources for prescribed uses.  Quite often, such basins will have Preferred Uses designated for future development to insure that the basin's Perennial Yield is not exceeded.  Also referred to as Designated Groundwater Basin.  Also see Designated Groundwater Basin (Nevada).
Adsorbate Any material adsorbed onto the surface of another.
Adsorbent Any material which adsorbs another on its surface.
Adsorber A solid or liquid that can hold molecules of another substance on its surface.
Adsorption (1) The adherence of ions or molecules in solution to the surface of solids.  (2) The adherence of a gas, liquid, or dissolved material on the surface of a solid.  (3) The attraction and adhesion of a layer of ions from an aqueous solution to the solid mineral surfaces with which it is in contact.  An example is the adsorption of organic materials by activated carbon.  Not to be confused with Absorption.
Advanced Treatment A level of wastewater treatment more stringent than secondary treatment; requires an 85 percent reduction in conventional pollutant concentration or a significant reduction in nonconventional pollutants.
Advanced Wastewater Treatment (AWT) Any process which reduces the level of impurities in a wastewater below that attainable through conventional secondary or biological treatment.  Includes the removal of nutrients such as phosphorus and nitrogen and a high percentage of suspended solids.  Also see Tertiary Wastewater Treatment.
Advance Time The time it takes for water to travel the length of an irrigation furrow.
Advection (1) The process by which solutes are transported by the bulk of flowing fluid such as the flowing ground water.  (2) The horizontal transfer of heat energy by large-scale motions of the atmosphere.
Aedile An elected official of ancient Rome who was responsible for public works and games and who supervised markets, the grain supply, and the water supply.
Aeolian Soil Soil transported from one area to another by the wind.
Aerate To supply or charge a liquid or body of water with a gas, as to expose a body of water to the circulation of air for purification.  See Aerated Lagoon.
Aerated Lagoon A holding and/or treatment pond that speeds up the natural process of biological decomposition of organic waste by stimulating the growth and activity of bacteria that degrade organic waste.
Aeration (1) Any active or passive process by which intimate contact between air and liquid is assured, generally by spraying liquid in the air, bubbling air through water, or mechanical agitation of the liquid to promote surface absorption of air.  (2) The process of loosening or puncturing the soil by mechanical menas in order to increase water and air permeability.
Aeration Tank A chamber used to inject air into water.
Aeration (Unsaturated) Zone The zone between the land surface and the water table which characteristically contains liquid water under less than atmospheric pressure and water vapor and air or other gases at atmospheric pressure.  The term Unsaturated Zone is now generally applied.
Aerobe An organism which requires oxygen for its life processes.
Aerobic Characterizing organisms able to live only in the presence of air or free oxygen, and conditions that exist only in the presence of air or free oxygen.  Contrast with Anaerobic.
Aerobic Bacteria Single-celled, microscopic organisms that require oxygen to live and are partly responsible for the Aerobic Decomposition of organic wastes.
Aerobic Decomposition The biodegradation of materials by aerobic microorganisms resulting in the production of carbon dioxide, water, and other mineral products.  Generally a faster process than Anaerobic Decomposition.  Also see Aerobic Bacteria.
Aerobic Treatment The process by which microbes decompose complex organic compounds in the presence of oxygen and use the liberated energy for reproduction and growth.  Such processes may include extended aeration, trickling filtration, and rotating biological contactors.
Aerobic Treatment Unit (ATU) Also known as home aeration units, the aerobic treatment unit, or ATU, provides wastewater treatment and storage functions similar to a normal septic tank.  By contrast, however, the ATU has a mechanism to inject air into the tank, thereby turning the ana
Aerodynamic Refers to forces acting upon the soil or crop surface by moving air.
Aeroponics A technique for growing plants without soil or hydroponic media.  The plants are held above a system that constantly mists the roots with nutrient-laden water.  Also called Aeroculture.
Aerosol A suspension of liquid or solid particles in air or gas.
Affected Environment (1) Existing biological, physical, social and economic conditions of an are subject to change, both directly and indirectly, as the result of a proposed human action.  (2) The chapter in an Environmental Impact Statement (EIS) describing current environmental conditions.
Affected Public The people who live and/or work near a hazardous waste site or other source of pollutant emissions.
Affluent (Stream) A stream or river that flows into a larger one; a Tributary.
Afforestation The artificial establishment of forest crops by planting or sowing on land that has not previously, or recently, grown trees.
Afterbay The tail race or reservoir of a hydroelectric power plant or a pumping plant at the outlet of the turbines used to regulate the flow below the plant; may refer to a short stretch of stream or conduit, or to a pond or reservoir.  Compare with Forebay.
Age (of Groundwater) An approximation of the time between the water's penetration of the land surface at one location and its later presence at another location.
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.
Aggressive Water Water which is soft and acidic and can corrode plumbing, piping, and appliances.
Agitated Pit A reservoir, pit, or pond that ordinarily is not stirred or aerated, but which is mixed just before emptying to suspend any settled solids.
Agitator/Mixer (Water Quality) Blades or paddles that slowly rotate in a tank to facilitate the mixing of suspended material.
Agribusiness The sum of all operations involved in the production, storage, processing, and wholesale marketing of agricultural products.
Agricultural Drainage (1) The process of directing excess water away from the root zones of plants by natural or artificial means, such as by using a system of pipes and drains placed below ground surface level.  Also referred to as Subsurface Drainage.  (2) The water drained away from irrigated farmland.
Agricultural Drought A general form of drought which occurs when soil moisture availability to agricultural crops is reduced to a level causing adverse effects on grain yield and consequently, the agricultural production of a region.  Compare to Hydrological Drought and Meteorological Drought.
Agricultural Economics The application of economic principles to the Agribusiness sector of the economy.
Agricultural Land Land in farms regularly used for agricultural production; all land devoted to crop or livestock enterprises, for example, farmstead lands, drainage and irrigation ditches, water supply, cropland, and grazing land.
Agricultural Levee A levee that protects agricultural areas where the degree of protection is usually less than that of a flood control levee.
Agricultural Pollution Liquid and solid wastes from all types of farming, including runoff from pesticides, fertilizers, and feedlots; erosion and dust from plowing; animal manure and carcasses; and crop residues and debris.  Agriculture is generally recognized as the leading nonpoint source of water pollutants, such as sediments, nutrients, and pesticides.  Among other water pollution problems related to agriculture, nitrate pollution has been of growing concern.  Nitrate contamination from agricultural activities has been reported in almost every state in the United States.
Agricultural Restructuring Scenario (ARS) A term used to describe the sensitivity of agricultural water demand and farm marketing revenues to changes in certain cropping patterns.
Agricultural Runoff The runoff into surface waters of herbicides, fungicides, insecticides, and the nitrate and phosphate components of fertilizers and animal wastes from agricultural land and operations.  Considered a Non-Point Source (NPS) of water pollution.
Agricultural Use The use of any tract of land for the production of animal or vegetable life; uses include, but are not limited to, the pasturing, grazing, and watering of livestock and the cropping, cultivation, and harvesting of plants.
Agricultural Water Use (Withdrawals) Includes water used for irrigation and non-irrigation purposes.  Irrigation water use includes the artificial application of water on lands to promote the growth of crops and pasture, or to maintain vegetative growth in recreational lands, parks, and golf courses.  Nonirrigation water use includes water used for livestock, which includes water for stock watering, feedlots, and dairy operations, and fish farming and other farm needs.
Agro-Ecosystem Land used for crops, pasture, and livestock; the adjacent uncultivated land that supports other vegetation and wildlife; and the associated atmosphere, the underlying soils, ground and surface waters, irrigation channels, and drainage networks.
Agroindustrial Of or relating to production (as of power for industry and water for irrigation) for both industrial and agricultural purposes..
Aground Onto or on a shore, reef, or the bottom of a body of water.
Air The colorless, odorless, tasteless, gaseous mixture that makes up the earth's Atmosphere.  Four gases comprise 99.997 percent (by volume) of clean, dry, air:  Nitrogen (78.084 percent); Oxygen (20.946 percent); Argon (0.934 percent); and Carbon Dioxide (0.033 percent).  The remaining components include neon, helium, methane, krypton, nitrous oxide, hydrogen, xenon, and various organic vapors.  Under normal conditions, air contains up to about 3 percent water vapor (by volume) and many solid, liquid, or gaseous contaminants introduced by human activities and natural causes such as wind erosion and the burning of fossil fuels.
Air Binding A situation where air enters the filter media and harms both the filtration and backwash processes.
Air-Bound Condition in a pipeline wherein air trapped in a summit prevents the free flow of the material in the pipeline.
Air Curtain A method for mechanical containment of oils spills in which air is bubbled through a perforated pipe, causing an upward water flow that retards the spreading of oil; also used as barriers to prevent fish from entering a polluted body of water.
Air Gap An open vertical gap or empty space that separates a drinking water supply to be protected from another water system in a treatment plant or other location.  The open gap protects the drinking water from contamination by backflow or backsiphonage.
Air Hole An opening in the frozen surface of a body of water.
Air Injection In groundwater management, the pumping of compressed air into the soil to move water in the Unsaturated Zone (Vadose Zone) down to the Saturated Zone (Phreatic Zone), or Water Table.
Air Lock A bubble or pocket of air or vapor, as in a pipe, that stops the normal flow of fluid through the conducting part.
Air Mass A large body of air of considerable depth which is approximately homogeneous horizontally.  At the same level, it has nearly uniform physical properties, especially as regards to temperature and moisture.
Air Padding Pumping dry air into a container to assist with the withdrawal of liquid or to force a liquefied gas such as chlorine out of the container.
Air Stripping (Water Quality) A process for the removal of organic contaminants from groundwater.  The groundwater flows downward inside a tower filled with materials (the packing) over a large surface area.  Air is introduced at the bottom of the tower and is forced upward past the falling water.  Individual organic contaminants are transferred from the water to the air, according to the gas and water equilibrium concentration values of each contaminant.  Also referred to as Packed Tower Aeration.
Air Vent (of a Dam) A pipe designed to provide air to the outlet conduit to reduce turbulence and prevent negative pressures during the release of water.  Extra air is usually necessary downstream of constrictions.
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.
Aldosterone A steroid hormone secreted by the adrenal cortex that regulates the salt and water balance in the human body.
Alfalfa Valve A screw-type valve placed on the end of a pipe to regulate the flow of water.
Algae Simple single-celled, colonial, or multi-celled, mostly aquatic plants, containing chlorophyll and lacking roots, stems and leaves.  Aquatic algae are microscopic plants that grow in sunlit water that contains phosphates, nitrates, and other nutrients.  Algae, like all aquatic plants, add oxygen to the water and are important in the fish food chain.
Algal Bloom (1) Rapid growth of algae on the surface of lakes, streams, or ponds; stimulated by nutrient enrichment.  (2) 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.  Also spelled Algae Bloom.
Algal Growth Potential (AGP) The maximum algal dry weight biomass produced in a natural water sample under laboratory conditions.  Expressed as milligrams (mg), dry weight per liter (l) of sample.
Algal Growth Rate A measure of algal productivity in a body of water, the growth rate measures the mass of carbon used annually by algae per unit area of lake surface.  The growth rate, typically referred to as Primary Productivity, is expressed as an index figure in grams of carbon per square meter per year, and indicates the state of Eutrophication of a body of water.  Algal productivity is influenced by the quantities of nutrients that flow into, or fall onto, the lake each year and the number of days of sunshine.  Another important factor is the mixing of the lake, which brings up to the surface where algae exist nutrients which have accumulated near the bottom of the lake.
Algicide One of a group of plant poisons used to kill filamentous algae and phytoplankton.
Algorithm A series of well-defined steps used in carrying out a specific process.  May be in the form of a word description, an explanatory note, a diagram or labeled flow chart, or a series of mathematical equations.
Alkali Any strongly basic (high pH) substance capable of neutralizing an acid, such as soda, potash, etc., that is soluble in water and increases the pH of a solution greater than 7.0.  Also refers to soluble salts in soil, surface water, or groundwater.
Alkaline Sometimes water or soils contain an amount of Alkali substances sufficient to raise the pH value above 7.0 and be harmful to the growth of crops.  Generally, the term alkaline is applied to water with a pH greater than 7.4.
Alkalinity (1) Refers to the extent to which water or soils contain soluble mineral salts.  Waters with a pH greater than 7.4 are considered alkaline.  (2) The capacity of water for neutralizing an acid solution.  Alkalinity of natural waters is due primarily to the presence of hydroxides, bicarbonates, carbonates and occasionally borates, silicates and phosphates.  It is expressed in units of milligrams per liter (mg/l) of CaCO3 (calcium carbonate).  A solution having a pH below 4.5 contains no alkalinity.
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.
Allopathy An interaction between plant species in which one species inhibits the establishment or growth on the second species through production of a selectively inhibitory chemical agent.
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 mountai
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.
Alluvial Flat A nearly level, graded, alluvial surface between the piedmont slope and playa of a Bolson or the axial-stream floodplain of a semi-bolson.  This major landform may include both recent and relict components.
Alluvial Land Areas of unconsolidated alluvium, generally stratified and varying widely in texture, recently deposited by streams, and subject to frequent flooding.
Alluvial Plain (1) A level or gently sloping tract or a slightly undulating land surface produced by extensive deposition of Alluvium, usually adjacent to a river that periodically overflows its banks; it may be situated on a Flood Plain, a delta, or an Alluvial Fan.  (2) A major landform of some basin floors, comprised of the floodplain of a major Pleistocene stream that crossed the floor, or of a low gradient fan-delta built by such a stream.  It is distinguished from an alluvial flat by its relatively well sorted and stratified alluvium.
Alluvial Valley Floor (Public Law 95-87, Section 701) (Legal) "The unconsolidated stream laid deposits where water availability is sufficient for subirrigation or flood irrigation.  It does not include upland areas which are generally overlain by a thin veneer of colluvial deposits composed chiefly of debris from sheet erosion, deposits by unconcentrated runoff or slopewash, talus, or other mass movement accumulation and wind-blown deposits."
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.
Alluvium (1) A general term for deposits of clay, silt, sand, gravel, or other particulate material that has been deposited by a stream or other body of running water in a streambed, on a flood plain, on a delta, or at the base of a mountain.  (2) A general term for such unconsolidated detrital material deposited during comparatively recent geologic time by a stream or other body of running water as a sorted or semi-sorted sediment in the bed of the stream or its flood plain or delta, or as a cone or fan at the base of a mountain slope; especially such a deposit of fine-grained texture (silt or silty clay) deposited during time of flood.  Also see Alluvion.
Alpenglow A rosy glow that suffuses snow-covered mountain peaks at dawn or dusk on a clear day.
Alpine That portion of mountains above tree growth; or organisms living there.
Alpine Decree (California and Nevada) The Federal Court adjudication of the relative water rights on the Carson River which is the primary regulatory control of Carson River operations today.  The decree is administered in the field by a Watermaster appointed by the federal district court.  T
Alternate Concentration Limits (ACLs) One of the three types of standards that may be applied when a leak is detected at a treatment, storage, or disposal facility and groundwater compliance monitoring is required.  ACLs are set by the U.S. Environmental Protection Agency (EPA) for specific hazardous waste constituents at levels that are designed to prevent a substantial hazard to human health or the environment.  Groundwater compliance monitoring can use the following standards:  (1) background concentrations, or the levels found in the area naturally; (2) specific values set by federal regulations in Title 40, Part 264.94, of the Code of Federal Regulations for eight metals and six pesticides and herbicides; or (3) alternative concentration limits.
Alternatives Courses of action which may meet the objectives of a proposal at varying levels of accomplishment, including the most likely future conditions without the project or action.
Altithermal (Climatology) A period of time when it was much warmer than now, approximately 7,000-4,500 years before the present time.  Also see Anathermal and Medithermal.
Altitude The vertical distance of a level, a point, or an object considered as a point, measured from Mean Sea Level (MSL).
Altocumulus Cloud A fleecy cloud, usually a rounded mass, but which can change radically and unexpectedly, producing intermediate forms, at an average height of 2.5 miles (4 kilometers).  Also see Cloud.
Altostratus Cloud A somewhat high level, blue to grayish blue cloud that forms a sheet or layer at an average height of 3.5 miles (5.6 kilometers).  Also see Cloud.
Altum Mare (Old English Law) The high seas; the deep sea.
Alum Common name for commercial-grade Aluminum Sulfate.  Its chemical formula is generally denoted by Al2(SO4)3 ? 12H2O (number of bound water molecules will vary from 12 to 18).
Aluminum Sulfate A white crystalline compound, Al2(SO4)3, used chiefly in paper making, water purification, sanitation, and tanning.  See Alum.
Ambient Water Quality Standards The allowable amount of materials, as a concentration of pollutants, in water.  The standard is set to protect against anticipated adverse effects on human health or welfare, wildlife, or the environment, with a margin of safety in the case of human health.  Also see Primary and Secondary Standards and 7Q10.
Amebic Dysentery A disorder of the gastrointestinal tract caused by a protozoan parasite belonging to the genus Entamoeba histolytica.  The disorder is commonly found in communities with poor sanitary conditions, particularly related to water and food storage and preparation.  Infected individuals experience abdominal cramps, diarrhea, and blood and mucus in the feces.  The parasite invades the liver in some cases.
American Public Works Association (APWA) A national organization founded in 1894 and based in Chicago, Illinois of individuals and organizations involved in the management of municipal solid waste and in the design and operation of wastewater treatment plants.
American Society of Civil Engineers (ASCE) A professional organization in New York City founded in 1852 that supports the practice of, and research in, environmental engineering, hydrology, and water and wastewater treatment.
American Water Works Association (AWWA) A national organization in Denver, Colorado, founded in 1881 of individuals involved in the design and operation of public water supplies and systems.
Amictic Lake A lake that does not experience mixing or turnover on a seasonal basis.  Also see Dimictic Lake.
Ammonia Stripping A process for the removal of ammonia from wastewater.  The waste is first made alkaline to favor the NH3 form, and then aerated so that exchange between the water and the atmosphere is encouraged.  Stripping towers are often used, with the waste trickling downward as air is forced upward through the tower.
Ammonification The transformation of organic nitrogen to ammonia, generally by means of bacterial activity.
Ammonium Sulfate A brownish-grey to white crystalline salt, (NH4)2SO4, used in fertilizers and water purification.
Amoeba, also Ameba A protozoan of the genus Amoeba or related genera, occurring in water and soil and as a parasite in other animals.  An amoeba has no definite form and consists essentially of a mass of protoplasm containing one nucleus or more surrounded by a delicate, flexible outer membrane.  It moves by means of pseudopods.
Amphibian (1) A cold-blooded, smooth-skinned vertebrate of the class Amphibia, such as a frog or salamander, that characteristically hatches as an aquatic larva with gills.  The larva then transforms into an adult having air-breathing lungs.  (2) An animal capable of living both on land and in water.
Amphibiotic Living in water during an early stage of development and on land during the adult stage.
Amphibious (Biology) (1) Living or able to live both on land and in water.  (2) Able to operate both on land and in water.
Ampoules A sealed, liquid-filled tube which is broken to release or be filled with another fluid.
Amprometric Titration A means to measure concentrations of certain substances in water using an electric current that flows during a chemical reaction.  Also see Titration.
Anabaena Any of various freshwater algae of the genus anabaena that sometimes occur in drinking water and cause a bad taste and odor.
Anabranch A diverging branch of a river which re-enters the main stream.
Anadromous Pertaining to fish that spend a part of their life cycle in the sea and return to freshwater streams to spawn, for example, salmon, steelhead trout, and shad.  Contrast with Catadromous.
Anaerobe An organism that does not require oxygen to maintain its life processes.
Anaerobic Characterizing organisms able to live and grow only where there is no air or free oxygen, and conditions that exist only in the absence of air or free oxygen.
Anaerobic Decomposition The degradation of materials by Anaerobic microorganisms living beneath the ground or in oxygen-depleted water to form reduced compounds such as methane or hydrogen sulfide.  Generally a slower process than Aerobic Decomposition.
Anaerobic Digester An airtight tank in which Anaerobic microorganisms decompose organic material and produce Biogas, mainly Methane.  Sewage treatment plants often use anaerobic digesters to reduce the volume of Sludge produced in Primary and Secondary Treatment, and they sometimes use the methane as a heating fuel.
Anaerobic Digestion The degradation of organic matter by microorganisms in the absence of oxygen, particularly as related to the treatment of sewage sludge.  Sewage treatment plants often use anaerobic digesters to reduce the volume of sludge produced in primary and secondary treatment, and they sometimes use the resultant methane gas as a heating fuel.
Anaho Island National Wildlife Refuge (NWR) (Nevada) One of the nine National Wildlife Refuges (NWR) located in the State of Nevada, the Anaho Island NWR was established in 1913 by President Woodrow Wilson to protect the white pelican nesting colonies.  The Anaho NWR consists of the 750-acre (1.2 square mile) Anaho Island located within Pyramid Lake, which is wholly contained within the Pyramid Lake Paiute Indian Reservation.  Situated approximately 45 northeast of Reno, Nevada, the Anaho NWR contains one of the largest white pelican nesting colonies in North America, as well as cormorant, great blue heron, and gull nesting colonies.  This refuge is closed to the public for the protection of the colony nesting birds.  Also see National Wildlife Refuge (NWR) System and National Wildlife Refuges (NWR) (Nevada).
Analog A continuously variable electrical signal representing a measured quantity.  For example, electrical signals such as current, voltage, frequency, or phase used to represent physical quantities such as water level, flow, and gate position.
Analytical Model A model that provides approximate or exact solutions to simplified forms of the differential equations for water movement and solute transport.  Such models generally require the use of complex calculations and the use of computers.
Anastomosing The branching and rejoining of channels to form a netlike pattern.
Anathermal (Climatology) The period preceding the Altithermal; the early Holocene epoch from about 10,000-7,000 years before the present.  Also see Medithermal.
Anchor A series of methods used to secure a structure to its footings or foundation wall so that it will not be displaced by flood or wind forces.
Anchor Ice Frazil ice that has collected on rocks on the stream bed.
Ancient Water Course A water course is said to be "ancient" if the channel through which it naturally runs has existed from time immemorial independent of the amount of water which it discharges.
Aneroid Not using liquid.
Angiosperms (Angiospermea) (Botanical) The vast majority of seed plants characterized as having ovules and seeds in a closed ovary.  Along with the Gymnosperms (Gymnospermae), Angiosperms comprise a structurally superior class within the plant family Spermatophyta, or seed plants.  Its two sub-classes consist of Monocotyledones and Dicotyledones.  Also see Gymnosperms.
Angler-Day The time spent fishing by one person for any part of a day.
Anhydride A chemical compound formed from another, often an acid, by the removal of water.
Anhydrous Without water, especially water of crystallization; not hydrated (Dehydrated).
Anion In an electrolyzed solution, the negatively charged particle, or ion, which travels to the anode and is therefore discharged, evolved, or deposited.  Also, by extension, any negative ion.
Anisotropy (1) The condition of having different properties in different directions.  (2) The condition under which one or more of the hydraulic properties of an aquifer vary according to the direction of the flow.
Annual Flood The highest peak discharge of a stream in a Water Year.
Annual Flood Series A list of annual floods for a given period of time.
Annual Low-Flow The lowest flow occurring each year, usually the lowest average flow for periods of perhaps 3, 7, 15, 30, 60, 120, or 180 consecutive days.
Annual Runoff The total quantity of water in runoff for a drainage area for the year.  Data reports may use any of the following units of measurement in presenting annual runoff data: (1) acre-feet (AC-FT, acre-ft, af)- the quantity of water required to cover 1 acre to a depth of 1 foot and is equal to 43,560 cubic feet, 325,851 gallons, or 1,234 cubic meters; (2) cubic feet per second per square mile (CFSM, (ft3/s)mi2) - the average number of cubic feet of water flowing per second from each square mile of area drained, assuming the runoff is distributed uniformly in time and area; (3) inch (In., in.) - the depth to which a drainage area would be covered with water if all the runoff for a given time period was uniformly distributed on it.
Annual 7-Day Minimum (USGS) The lowest mean discharge for 7 consecutive days in a year.  Note that most low-flow frequency analyses of annual 7-day minimum flows use a climatic year (i.e., April 1-March 31).  The date shown in USGS statistical tables is the initial date of the 7-day period.  This value should not be confused with the 7-day 10-year low-flow statistic.
Annular Space The space between two cylindrical objects, one of which surrounds the other, such as the space between the wall of the drilled hole and the casing, or between a permanent casing and the borehole.
Annulus For a well, the space between the pipe and the outer wall (casing) of the borehole, which may be a pipe also (the well casing).
Annulus Pressure The positive pressure maintained by a fluid introduced between the well piping and the outer wall (casing) of the borehole of an underground Injection Well providing an indication of the integrity of the well.
Anoxia (1) Absence of oxygen.  (2)  The total deprivation of oxygen, as in bodies of water, lake sediments, or sewage.
Anoxia, Functional Although not well defined, generally refers to a body of water sufficiently deprived of oxygen to where Zooplankton and fish would not survive.
Anoxic (1) Denotes the absence of oxygen, as in a body of water.  (2) Of, relating to, or affected with anoxia; greatly deficient in oxygen; oxygenless as with water.
Antecedent Moisture The degree of wetness of soil at the beginning of a runoff, determined by summation of weighted daily rainfall amounts for a period preceding the runoff.
Antecedent Moisture Condition (AMC) (1) A description of the amount of water in storage at some point in time (usually the start of a hydrologic event) that is relevant to the event.  (2) Soil moisture at the onset of a rainfall event.  The U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service (SCS), defines AMC in terms of total rainfall during the 5 days immediately preceding the rainfall event.  Dry AMC conditions mean less than 1.4 inches, average is 1.4 to 2.1 inches, and wet is greater than 2.1 inches.
Antecedent Precipitation Precipitation which occurred prior to a particular time over a specific area or Drainage Basin.  Usually applied as a measure of moisture in the top layer of the soil which would affect runoff from additional rainfall.  Also see Antecedent Precipitation Index (API).
Antecedent Precipitation Index (API) An index of moisture stored in a basin before a storm, calculated as a weighted summation of past daily precipitation amounts.  Also see Antecedent Precipitation.
Antecedent Soil Water Degree of wetness of a soil prior to irrigation or at the beginning of a runoff period, typically expressed as an index.
Antecedent Streams Antecedent streams are those in place before the rising of mountain chains.  As the mountains rise, the streams cut through at the same rate and so maintain their positions.
Antediluvian (Policy) (1) Extremely old and antiquated.  (2) Occurring or belonging to the era before the Flood written about in the Bible.  (Ecology) Used sometimes today to denote a public growth and water policy based on an area's natural ability to support population growth only through existing, readily available natural resources, i.e., water.
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.
Anticyclone An area of relatively high pressure in which, in the northern hemisphere, the winds tend to blow spirally outward in a clockwise direction.
Antidegradation Policy (or Clause) Rules or guidelines that are required of each state by federal regulations implementing the Clean Water Act (CWA), stating that existing water quality be maintained even if the current water quality in an area is higher than the minimum permitted as defined by federal ambient water quality standards.  Some controlled degradation is permitted in support of economic development.
Antifreeze A substance, often a liquid such as ethylene glycol or alcohol, mixed with another liquid, such as water, to lower its freezing point.
Antifluoridationist One who is strongly opposed to the fluoridation of public water supplies.
Anti-Seepage Collar A projecting collar, usually of concrete, built around the outside of a pipe, tunnel, or conduit, or conduit under or through an Embankment Dam to lengthen the seepage path along the outer surface of the conduit.
Apex (1) The highest point on an Alluvial Fan or similar landform below which the flow path of the major stream that formed the fan becomes unpredictable and Alluvial Fan Flooding can occur.  (2) The point of highest elevation on an alluvial fan, which on undisturbed fans is generally the point where the major stream that formed the fan emerges from the mountain front.
Aphotic Defined as without light.  Of or relating to the region of a body of water that is not reached by sunlight and in which Photosynthesis is unable to occur.  The Aphotic Zone of the ocean is the water deeper than about 800 meters (2,625 feet), beyond which no light penetrates.  Contrast with Photic Zone.
Apothecaries' Measure A system of liquid volume measure used in pharmacy.
Applicable or Appropriate Requirements (ARARs) Any state or federal statute that pertains to the protection of human life and the environment in addressing specific conditions or use of a particular cleanup technology at a Superfund Site.
Application Rate For irrigation, the rate at which water is applied per unit of land area, usually expressed in terms of inches per hour.
Application, Water Right An official request for permission to develop a source of water or to change an existing water right; includes a description of the proposed project, a map of the project, and a legal description of the property involved.  The application for a water righ
Applied Water Demand The quantity of water delivered to the intake of a city's water system or factory, the farm headgate or other point of measurement, or a marsh or other wetland, either directly or by incidental drainage.  For instream use, it is the portion of the stream flow dedicated to instream use or reserved under federal or state legislation such as Wild and Scenic River Acts.  Applied water includes the water that returns to groundwater, a stream, canal, or other supply source that can be reused or recycled and thus is not the same as Net Water Demand.
Appropriate To authorize the use of a quantity of water to an individual requesting it.
Appropriated Water (1) A quantity of water from a well, stream, river, reservoir, or other source reserved for a specific use and place of use under state water-right laws, statutes, or regulations.  (2) Surface water in an irrigation district that has been assigned or allocated to owners of water rights.
Appropriate Technology The application of current scientific knowledge and technology is such a way so as to conform with existing economic, infrastructure, social, and cultural conditions and practices.  By extension, the concept implies the implementation of low-technology solutions incorporating simplicity of design, use, and maintenance.
Appropriation A granting process whereby authority is granted by a state to divert, store, or use the public waters of the state.  Often used interchangeably with the terms water right and water permit.
(Prior) Appropriation Doctrine The system for allocating water to private individuals used in the western United States under which (1) the right to water was acquired by diverting water and applying it to a beneficial use and (2) a right to water acquired earlier in time is superior t
Appropriative Water Right (Nevada) Nevada's water law is based on statutes enacted in 1903 and 1905 and are founded on the principal of Prior Appropriation.  Unlike some other states, Nevada has a statewide system for the administration of both ground water and surface water.  Appropriativ
Appropriator One taking water from a watercourse under the authority of the state and applying it to Beneficial Use.
Approximate Original Contour The surface configuration achieved by backfilling and grading of mined areas so that the reclaimed area, including any terracing or access roads, closely resembles the general surface configuration of the land prior to strip mining and blends into and complements the drainage pattern of the surrounding terrain.
Appurtenant (1) (Legal) A right, privilege, or property that is considered incident to the principal property for purposes such as passage of title, conveyance, or passage of title.  (2) (Water-Related) A right to water that is incident to the ownership or possession of the land.
Appurtenant Land The land base to which water rights legally pertain or belong.