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Washington Water Science Center Glossary

0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z



The process by which substances in gaseous, liquid, or solid form are assimilated or taken up by other substances.


Has a pH of water less than 5.5; pH modifier used in the U.S. Fish and Wildlife Service wetland classification system; in common usage, acidic water has a pH less than 7.

Acidic deposition

The transfer of acidic or acidifying substances from the atmosphere to the surface of the Earth or to objects on its surface. Transfer can be either by wet-deposition processes (rain, snow, dew, fog, frost, hail) or by dry deposition (gases, aerosols, or fine to coarse particles).


The volume of water needed to cover an acre of land to a depth of one foot; equivalent to 43,560 cubic feet or 325,851 gallons.


The adherence of gas molecules, ions, or molecules in solution to the surface of solids.


To supply air to water, soil, or other media.


Pertaining to, taking place in, or caused by the presence of oxygen.


Chlorophyll-bearing nonvascular, primarily aquatic species that have no true roots, stems, or leaves; most algae are microscopic, but some species can be as large as vascular plants.

Algal bloom

The rapid proliferation of passively floating, simple plant life, such as blue-green algae, in and on a body of water.


Has a pH greater than 7; pH modifier in the U.S. Fish and Wildlife Service wetland classification system; in common usage, a pH of water greater than 7.4.

Alluvial aquifer

A water-bearing deposit of unconsolidated material (sand and gravel) left behind by a river or other flowing water.


General term for sediments of gravel, sand, silt, clay, or other particulate rock material deposited by flowing water, usually in the beds of rivers and streams, on a flood plain, on a delta, or at the base of a mountain.

Alpine snow glade

A marshy clearing between slopes above the timberline in mountains.


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


A compound of nitrogen and hydrogen (NH3) that is a common by-product of animal waste. Ammonia readily converts to nitrate in soils and streams.

Anadromous fish

Migratory species that are born in freshwater, live mostly in estuaries and ocean water, and return to freshwater to spawn.


Pertaining to, taking place in, or caused by the absence of oxygen.


As related to fish, externally visible skin or subcutaneous disorders, including deformities, eroded fins, lesions, and tumors.


Having to do with or caused by humans.


A fold in the Earth's crust, convex upward, whose core contains stratigraphically older rocks.


The science of farming organisms that live in water, such as fish, shellfish, and algae.


Living or growing in or on water.

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. Commonly refers to criteria established by the U.S. Environmental Protection Agency. See also Water-quality guidelines, Water-quality criteria, and Freshwater chronic criteria.


A geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to springs and wells.


A small, deep, flat-floored channel or gully of an ephemeral or intermittent stream, usually with nearly vertical banks cut, into unconsolidated material. A term commonly used in the arid and semiarid regions of the Southwestern United States.

Artificial recharge

Augmentation of natural replenishment of ground-water storage by some method of construction, spreading of water, or by pumping water directly into an aquifer.

Atmospheric deposition

The transfer of substances from the air to the surface of the Earth, either in wet form (rain, fog, snow, dew, frost, hail) or in dry form (gases, aerosols, particles).

Atmospheric pressure

The pressure exerted by the atmosphere on any surface beneath or within it; equal to 14.7 pounds per square inch at sea level.

Average discharge

As used by the U.S. Geological Survey, the arithmetic average of all complete water years of record of surface water discharge whether consecutive or not. The term "average" generally is reserved for average of record and "mean" is used for averages of shorter periods, namely, daily, monthly, or annual mean discharges.


Background concentration

A concentration of a substance in a particular environment that is indicative of minimal influence by human (anthropogenic) sources.


A body of water in which the flow is slowed or turned back by an obstruction such as a bridge or dam, an opposing current, or the movement of the tide.


Single-celled microscopic organisms.


The sloping ground that borders a stream and confines the water in the natural channel when the water level, or flow, is normal.

Bank storage

The change in the amount of water stored in an aquifer adjacent to a surface-water body resulting from a change in stage of the surface-water body.

Barrier bar

An elongate offshore ridge, submerged at least at high tide, built up by the action of waves or currents.

Barrier beach

A narrow, elongate sandy ridge rising slightly above the high-tide level and extending generally parallel with the mainland shore, but separated from it by a lagoon.

Base flow

The sustained low flow of a stream, usually ground-water inflow to the stream channel.


The opposite of acidic; water that has a pH of greater than 7.

Basic Fixed Sites

Sites on streams in NAWQA Study Units at which streamflow is measured and samples are collected for analysis of 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 streamwater in relation to hydrologic conditions and environmental settings.

Basin and Range physiography

A region characterized by a series of generally north-trending mountain ranges separated by alluvial valleys.

Bed material

Sediment composing the streambed.

Bed sediment

The material that temporarily is stationary in the bottom of a stream or other watercourse.

Bed sediment and tissue studies

Assessment of concentrations and distributions of trace elements and hydrophobic organic contaminants in streambed sediment and tissues of aquatic organisms to identify potential sources and to assess spatial distribution of those constituents.


Sediment that moves on or near the streambed and is in almost continuous contact with the bed.


A general term used for solid rock that underlies soils or other unconsolidated material.

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 organism

A form of aquatic life that lives on or near the bottom of streams, lakes, or oceans.

Best management practice

An agricultural practice that has been determined to be an effective, practical means of preventing or reducing nonpoint-source pollution.


To exert a strong chemical attraction.


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.


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


Refers to chemical processes that occur inside or are mediated by living organisms.

Biochemical process

A process characterized by, produced by, or involving chemical reactions in living organisms.

Biochemical-oxygen demand

The amount of oxygen, expressed in milligrams per liter, that is removed from aquatic environments by the life processes of micro-organisms.


Transformation of a substance into new compounds through biochemical reactions or the actions of microorganisms such as bacteria.


The amount of living matter, in the form of organisms, present in a particular habitat, usually expressed as weight-per-unit area.


All living organisms of an area.


A small saucer- or trough-shaped hollow or depression formed by wind erosion on a pre-existing dune or other sand deposit.

Blue-baby syndrome

A condition most common in young infants and certain elderly people that can be caused by ingestion of high amounts of nitrate, which results in the blood losing its ability to effectively carry oxygen.


A nutrient-poor, acidic wetland dominated by a waterlogged, spongy mat of sphagum moss that ultimately forms a thick layer of acidic peat; generally has no inflow or outflow; fed primarily by rain water.


An extensive, flat, saucer-shaped, alluvium-floored basin or depression, almost or completely surrounded by mountains and from which drainage has no surface outlet; a term used in the desert regions of the Southwestern United States.


A climatic zone having a definite winter with snow and a short summer that is generally hot, and which is characterized by a large annual range of temperature.


A dense growth of trees and underbrush.

Bottom-land forest

Low-lying forested wetland found along streams and rivers, usually on alluvial flood plains.

Brackish water

Water with a salinity intermediate between seawater and freshwater (containing from 1,000 to 10,000 milligrams per liter of dissolved solids).

Braided stream

A stream characterized by an interlacing or tangled network of several small branching and reuniting shallow channels.

Breakdown product

A compound derived by chemical, biological, or physical action upon a pesticide. The breakdown is a natural process that may result in a more toxic or a less toxic compound and a more persistent or less persistent compound.


Water that contains more than 35,000 milligrams per liter of dissolved solids.



A rock or substance formed of calcium carbonate or magnesium carbonate by biological deposition or inorganic precipitation, or containing those minerals in sufficient quantities to effervesce when treated with cold hydrochloric acid.


A large, more or less circular, basin-shaped volcanic depression whose diameter is many times greater than the volcanic vent.

Canopy angle

Generally, a measure of the openness of a stream to sunlight. Specifically, the angle formed by an imaginary line from the highest structure (for example, tree, shrub, or bluff) on one bank to eye level at midchannel to the highest structure on the other bank.

Capillary fringe

The zone above the water table in which water is held by surface tension. Water in the capillary fringe is under a pressure less than atmospheric.

Carbonate rocks

Rocks (such as limestone or dolostone) that are composed primarily of minerals (such as calcite and dolomite) containing the carbonate ion (CO32-).

Center pivot irrigation

An automated sprinkler system involving a rotating pipe or boom that supplies water to a circular area of an agricultural field through sprinkler heads or nozzles.

Channel scour

Erosion by flowing water and sediment on a stream channel; results in removal of mud, silt, and sand on the outside curve of a stream bend and the bed material of a stream channel.


The straightening and deepening of a stream channel to permit the water to move faster or to drain a wet area for farming.


Octachloro-4,7-methanotetrahydroindane. An organochlorine insecticide no longer registered for use in the U.S. Technical chlordane is a mixture in which the primary components are cis- and trans-chlordane, cis- and trans-nonachlor, and heptachlor.

Chlorinated solvent

A volatile organic compound containing chlorine. Some common solvents are trichloroethylene, tetrachloroethylene, and carbon tetrachloride.


A class of volatile compounds consisting of carbon, chlorine, and fluorine. Commonly called freons, which have been in refrigeration mechanisms, as blowing agents in the fabrication of flexible and rigid foams, and, until banned from use several years ago, as propellants in spray cans.


A marshy area where the ground is wet due to the presence of seepage or springs.


Said of water with a pH between 5.5 and 7.4; pH modifier used in the U.S. Fish and Wildlife Service wetland classification system.


A deep, steep-walled, half-bowllike recess or hollow situated high on the side of a mountain and commonly at the head of a glacial valley; and produced by the erosive activity of mountain glaciers.


Rock, such as sandstone, or sediment composed principally of broken fragments that are derived from preexisting rocks which have been transported from their place of origin.


The sum total of the meteorological elements that characterize the average and extreme conditions of the atmosphere over a long period of time at any one place or region of the Earth's surface.

Combined sewer overflow

A discharge of untreated sewage and stormwater to a stream when the capacity of a combined storm/sanitary sewer system is exceeded by storm runoff.

Commercial withdrawals

Water for use by motels, hotels, restaurants, office buildings, commercial facilities, and civilian and military institutions. The water may be obtained from a public supplier or it may be self-supplied.


In ecology, the species that interact in a common area.


The ratio of the quantity of any substance present in a sample of a given volume or a given weight compared to the volume or weight of the sample.

Cone of depression

The depression of heads around a pumping well caused by withdrawal of water.

Confined aquifer

An aquifer that is completely filled with water under pressure and that is overlain by material that restricts the movement of water.

Confining layer

A body of impermeable or distinctly less permeable (see permeability) material stratigraphically adjacent to one or more aquifers that restricts the movement of water into and out of the aquifers.


The flowing together of two or more streams; the place where a tributary joins the main stream.


A coarse-grained sedimentary rock composed of fragments larger than 2 millimeters in diameter.


A chemical or biological substance in water, sediment, or biota that can be measured by an analytical method.

Consumptive use

The quantity of water that is not available for immediate reuse because it has been evaporated, transpired, or incorporated into products, plant tissue, or animal tissue. Also referred to as "water consumption".

Contact recreation

Recreational activities, such as swimming and kayaking, in which contact with water is prolonged or intimate, and in which there is a likelihood of ingesting water.


Degradation of water quality compared to original or natural conditions due to human activity.

Contributing area

The area in a drainage basin that contributes water to streamflow or recharge to an aquifer.

Coral reef

A ridge of limestone, composed chiefly of coral, coral sands, and solid limestone resulting from organic secretion of calcium carbonate; occur along continents and islands where the temperature is generally above 18° C.

Core sample

A sample of rock, soil, or other material obtained by driving a hollow tube into the undisturbed medium and withdrawing it with its contained sample.


A standard rule or test on which a judgment or decision can be based.

Crystalline rocks

Rocks (igneous or metamorphic) consisting wholly of crystals or fragments of crystals.

Cubic foot per second

Rate of water discharge representing a volume of 1 cubic foot passing a given point during 1 second, equivalent to approximately 7.48 gallons per second or 448.8 gallons per minute or 0.02832 cubic meter per second. In a stream channel, a discharge of 1 cubic foot per second is equal to the discharge at a rectangular cross section, 1 foot wide and 1 foot deep, flowing at an average velocity of 1 foot per second.


An area of low pressure around which winds rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. See also Tropical cyclone.

Cypress dome

Small, isolated, circular, depressional, forested wetlands, in which cypress predominates, that have convex silhouettes when viewed from a distance.



Dichloro-diphenyl-trichloroethane. An organochlorine insecticide no longer registered for use in the United States.

Datum plane

A horizontal plane to which ground elevations or water surface elevations are referenced.


Refers to plants that shed foliage at the end of the growing season.

Deepwater habitat

Permanently flooded lands lying below the deepwater boundary of wetlands.

Degradation products

Compounds resulting from transformation of an organic substance through chemical, photochemical, and/or biochemical reactions.


Condition of the quality of water that has been made unfit for some specified purpose.


The low, nearly flat tract of land at or near the mouth of a river, resulting from the accumulation of sediment supplied by the river in such quantities that it is not removed by tides, waves, or currents. Commonly a triangular or fan-shaped plain.


A process by which oxidized forms of nitrogen such as nitrate (NO3-) are reduced to form nitrites, nitrogen oxides, ammonia, or free nitrogen: commonly brought about by the action of denitrifying bacteria and usually resulting in the escape of nitrogen to the air.


To determine the presence of a compound.

Detection limit

The concentration of a constituent or analyte below which a particular analytical method cannot determine, with a high degree of certainty, the concentration.


Single-celled, colonial, or filamentous algae with siliceous cell walls constructed of two overlapping parts.


An organochlorine insecticide no longer registered for use in the United States. Also a degradation product of the insecticide aldrin.

Direct runoff

The runoff entering stream channels promptly after rainfall or snowmelt.


The volume of fluid passing a point per unit of time, commonly expressed in cubic feet per second, million gallons per day, gallons per minute, or seconds per minute per day.

Discharge area

(Groundwater) Area where subsurface water is discharged to the land surface, to surface water, or to the atmosphere.


The extent to which a liquid substance introduced into a ground-water system spreads as it moves through the system.


Cut by erosion into valleys, hills, and upland plains.

Dissolved constituent

Operationally defined as a constituent that passes through a 0.45-micrometer filter.

Dissolved oxygen

Oxygen dissolved in water; one of the most important indicators of the condition of a water body. Dissolved oxygen is necessary for the life of fish and most other aquatic organisms.

Dissolved solids

Minerals and organic matter dissolved in water.


A turning aside or alteration of the natural course of a flow of water, normally considered physically to leave the natural channel. In some States, this can be a consumptive use direct from another stream, such as by livestock watering. In other States, a diversion must consist of such actions as taking water through a canal, pipe, or conduit.


A sedimentary rock consisting chiefly of magnesium carbonate.

Domestic withdrawals

Water used for normal household purposes, such as drinking, food preparation, bathing, washing clothes and dishes, flushing toilets, and watering lawns and gardens. The water may be obtained from a public supplier or may be self-supplied. Also called residential water use.

Dominant plant

The plant species controlling the environment.

Drainage area

The drainage area of a stream at a specified location is that area, measured in a horizontal plane, which is enclosed by a drainage divide.

Drainage basin

The land area drained by a river or stream.

Drainage divide

Boundary between adjoining drainage basins.


The difference between the water level in a well before pumping and the water level in the well during pumping. Also, for flowing wells, the reduction of the pressure head as a result of the discharge of water.

Drinking-water standard or guideline

A threshold concentration for a constituent or compound in a public drinking-water supply, designed to protect human health. As defined here, standards are U.S. Environmental Protection Agency regulations that specify the maximum contamination levels for public water systems required to protect the public welfare; guidelines have no regulatory status and are issued in an advisory capacity.

Drip irrigation

An irrigation system in which water is applied directly to the root zone of plants by means of applicators (orifices, emitters, porous tubing, or perforated pipe) operated under low pressure. The applicators can be placed on or below the surface of the ground or can be suspended from supports.


A prolonged period of less-than-normal precipitation such that the lack of water causes a serious hydrologic imbalance.


EPT richness index

An index based on the sum of the number of taxa in three insect orders, Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies), that are composed primarily of species considered to be relatively intolerant to environmental alterations.

Ecological studies

Studies of biological communities and habitat characteristics in NAWQA Study Units to evaluate the effects of physical and chemical characteristics of water and hydrologic conditions on aquatic biota and to determine how biological and habitat characteristics differ among environmental settings.


An area of similar climate, landform, soil, potential natural vegetation, hydrology, or other ecologically relevant variables.


A community of organisms considered together with the nonliving factors of its environment.


Outflow from a particular source, such as a stream that flows from a lake or liquid waste that flows from a factory or sewage-treatment plant.

Emergent plants

Erect, rooted, herbaceous plants that may be temporarily or permanently flooded at the base but do not tolerate prolonged inundation of the entire plant.

Endangered species

A species that is in imminent danger of becoming extinct.

Endocrine system

The collection of ductless glands in animals that secrete hormones, which influence growth, gender and sexual maturity.


The sum of all conditions and influences affecting the life of organisms.

Environmental framework

Natural and human-related features of the land and hydrologic system, such as geology, land use, and habitat, that provide a unifying framework for making comparative assessments of the factors that govern water-quality conditions within and among NAWQA Study Units.

Environmental sample

A water sample collected from an aquifer or stream for the purpose of chemical, physical, or biological characterization of the sampled resource.

Environmental setting

Land area characterized by a unique combination of natural and human-related factors, such as row-crop cultivation or glacial-till soils.

Ephemeral stream

A stream or part of a stream that flows only in direct response to precipitation; it receives little or no water from springs, melting snow, or other sources; its channel is at all times above the water table.

Equal-width increment sample

A composite sample of water collected across a section of stream with equal spacing between verticals and equal transit rates within each vertical that yields a representative sample of stream conditions.


The process whereby materials of the Earth's crust are loosened, dissolved, or worn away and simultaneously moved from one place to another.

Estuarine wetlands

Tidal wetlands in low-wave-energy environments where the salinity of the water is greater than 0.5 part per thousand and is variable owing to evaporation and the mixing of seawater and freshwater; tidal wetlands of coastal rivers and embayments, salty tidal marshes, mangrove swamps, and tidal flats.


Area where the current of a stream meets the ocean and where tidal effects are evident; an arm of the ocean at the lower end of a river.


The process by which water becomes enriched with plant nutrients, most commonly phosphorus and nitrogen.


he process by which water is changed to gas or vapor; occurs directly from water surfaces and from the soil.

Evaporite minerals

Minerals or deposits of minerals formed by evaporation of water containing salts. These deposits are common in arid climates.


A class of sedimentary rocks composed primarily of minerals precipitated from a saline solution as a result of extensive or total evaporation of water.


The process by which water is discharged to the atmosphere as a result of evaporation from the soil and surface-water bodies, and transpiration by plants.

Exotic species

Plants or animals not native to the area.


FDA action level

A regulatory level recommended by the U.S. Environmental Protection Agency for enforcement by the Food and Drug Administration (FDA) when pesticide residues occur in food commodities for reasons other than the direct application of the pesticide. Action levels are set for inadvertent pesticide residues resulting from previous legal use or accidental contamination. Applies to edible portions of fish and shellfish in interstate commerce.

Fall line

Imaginary line marking the boundary between the ancient, resistant crystalline rocks of the Piedmont province of the Appalachian Mountains, and the younger, softer sediments of the Atlantic Coastal Plain province in the Eastern United States. Along rivers, this line commonly is reflected by waterfalls.


Cropland, tilled or untilled, allowed to lie idle during the whole or greater part of the growing season.

Fecal bacteria

Microscopic single-celled organisms (primarily fecal coliforms and fecal streptococci) found in the wastes of warm-blooded animals. Their presence in water is used to assess the sanitary quality of water for body-contact recreation or for consumption. Their presence indicates contamination by the wastes of warm-blooded animals and the possible presence of pathogenic (disease producing) organisms.

Fecal coliform

Microscopic single-celled organisms (primarily fecal coliforms and fecal streptococci) found in the wastes of warm-blooded animals. Their presence in water is used to assess the sanitary quality of water for body-contact recreation or for consumption. Their presence indicates contamination by the wastes of warm-blooded animals and the possible presence of pathogenic (disease producing) organisms.


Peat-accumulating wetland that generally receives water from surface runoff and (or) seepage from mineral soils in addition to direct precipitation; generally alkaline; or slightly acid.


Any of a large number of natural or synthetic materials, including manure and nitrogen, phosphorus, and potassium compounds, spread on or worked into soil to increase its fertility.


Liquid that has been passed through a filter.

Fixed Sites

Monitoring sites in NAWQA Study Units at which the most comprehensive suites of data are collected. See also Basic Fixed Sites and Intensive Fixed Sites.


Any relatively high streamflow that overflows the natural or artificial banks of a stream.

Flood attenuation

a weakening or reduction in the force or intensity of a flood.

Flood irrigation

The application of irrigation water whereby the entire surface of the soil is covered by ponded water.

Flood plain

A strip of relatively flat land bordering a stream channel that is inundated at times of high water.

Flow line

The idealized path followed by particles of water.


An underground route for ground-water movement, extending from a recharge (intake) zone to a discharge (output) zone such as a shallow stream.


Pertaining to a river or stream.

Fluvial deposit

A sedimentary deposit consisting of material transported by suspension or laid down by a river or stream.


A specific air route taken by birds during migration.


Water that contains less than 1,000 milligrams per liter of dissolved solids.

Freshwater chronic criteria

The highest concentration of a contaminant that freshwater aquatic organisms can be exposed to for an extended period of time (4 days) without adverse effects.


Descriptive of a rock or mineral that crumbles naturally or is easily broken, pulverized, or reduced to powder.


A substance or mixture of substances that produces gas, vapor, fume, or smoke intended to destroy insects, bacteria, or rodents.

Furrow irrigation

A type of surface irrigation whereby water is applied at the upper (higher) end of a field and flows in furrows to the lower end.


Gage height

Height of the water surface above an established datum plane, such as in a river above a predetermined point that may (or may not) be at the channel floor.

Gaging station

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


Pertaining to the form or general configuration of the Earth or of its surface features.


The science that treats the general configuration of the Earth's surface; the description of landforms.


Relating to the Earth's internal heat; commonly applied to springs or vents discharging hot water or steam.


Of or relating to the presence and activities of ice or glaciers.

Glacial drift

A general term for rock material transported by glaciers or icebergs and deposited directly on land or in the sea.

Glacial lake

A lake that derives its water, or much of its water, from the melting of glacial ice; also a lake that occupies a basin produced by glacial erosion.

Glacial outwash

Stratified detritus (chiefly sand and gravel) "washed out" from a glacier by meltwater streams and deposited in front of or beyond the end moraine or the margin of an active glacier.

Granite rock

A coarse-grained igneous rock.

Ground water

In the broadest sense, all subsurface water; more commonly that part of the subsurface water in the saturated zone.

Ground-water flow system

The underground pathway by which ground water moves from areas of recharge to areas of discharge.

Growing season

The frost-free period of the year.



The part of the physical environment in which a plant or animal lives.


A property of water that causes the formation of an insoluble residue when the water is used with soap and a scale in vessels in which water has been allowed to evaporate. It is due primarily to the presence of ions of calcium and magnesium. Generally expressed as milligrams per liter as calcium carbonate (CaCO3).


The source and upper part of a stream.

Health advisory

Nonregulatory levels of contaminants in drinking water that may be used as guidance in the absence of regulatory limits. Advisories consist of estimates of concentrations that would result in no known or anticipated health effects (for carcinogens, a specified cancer risk) determined for a child or for an adult for various exposure periods.


With characteristics of an herb; a plant with no persistent woody stem above ground.


A type of pesticide designed to kill plants.

Human health advisory

Guidance provided by U.S. Environmental Protection Agency, State agencies or scientific organizations, in the absence of regulatory limits, to describe acceptable contaminant levels in drinking water or edible fish.

Hydraulic conductivity

The capacity of a rock to transmit water. It is expressed as the volume of water at the existing kinematic viscosity that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow.

Hydraulic gradient

The change of hydraulic head per unit of distance in a given direction.

Hydraulic head

The height of the free surface of a body of water above a given point beneath the surface.

Hydric soil

Soil that is wet long enough to periodically produce anaerobic conditions, thereby influencing the growth of plants.


Graph showing variation of water elevation, velocity, streamflow, or other property of water with respect to time.

Hydrologic cycle

The circulation of water from the sea, through the atmosphere, to the land, and thence back to the sea by overland and subterranean routes.

Hydrologic regime

The characteristic behavior and total quantity of water involved in a drainage basin.

Hydrologic unit

A geographic area representing part or all of a surface drainage basin or distinct hydrologic feature as delineated by the U. S. Geological Survey on State Hydrologic Unit Maps. Each hydrologic unit is assigned a hierarchical hydrologic unit code consisting of 2 digits for each successively smaller drainage basin unit.


The science that deals with water as it occurs in the atmosphere, on the surface of the ground, and underground.


Not capable of uniting with or absorbing water.


Any plant growing in water or on a substrate that is at least periodically deficient in oxygen as a result of excessive water content.

Hydrostatic pressure

The pressure exerted by the water at any given point in a body of water at rest.


Igneous rocks

Rocks that have solidified from molten or partly molten material.


To hold by a strong chemical attraction.


Condition of the quality of water that has been adversely affected for a specific use by contamination or pollution.


The incapacity of a rock to transmit a fluid.


Impermeable. The incapacity of a rock to transmit a fluid.

Index of Biotic Integrity

An aggregated number, or index, based on several attributes or metrics of a fish community that provides an assessment of biological conditions.

Indicator sites

Stream sampling sites (in NAWQA Study Units) located at outlets of drainage basins with relatively homogeneous land use and physiographic conditions; most indicator-site basins have drainage areas ranging from 20 to 200 square miles.


Cemented, hardened, or a rocklike condition.

Industrial withdrawals

Water withdrawn for or used for thermoelectric power (electric utility generation) and other industrial and manufacturing uses such as steel, chemical and allied products, paper and allied products, mining, and petroleum refining. The water may be obtained from a public supplier or may be self-supplied.


The downward movement of water from the atmosphere into soil or porous rock.


Containing no carbon; matter other than plant or animal.

Inorganic soil

Soil with less than 20 percent organic matter in the upper 16 inches.


A substance or mixture of substances intended to destroy or repel insects.

Instantaneous discharge

The volume of water that passes a point at a particular instant of time.

Instream use

Water use taking place within the stream channel for such purposes as hydroelectric power generation, navigation, water-quality improvement, fish propagation, and recreation. Sometimes called nonwithdrawal use or in-channel use.

Integrated drainage

Drainage developed during geomorphic maturity in an arid region, characterized by coalescence of drainage basins as a result of headward erosion in the lower basins or spilling over from the upper basins.

Integrator or Mixed-use site

Stream sampling site (in a NAWQA Study Unit) located at an outlet of a drainage basin that contains multiple environmental settings. Most integrator sites are on major streams with relatively large drainage areas.

Intensive Fixed Sites

Basic Fixed Sites with increased sampling frequency during selected seasonal periods and analysis of dissolved pesticides for 1 year. Most NAWQA Study Units have one to two integrator Intensive Fixed Sites and one to four indicator Intensive Fixed Sites


In hydrology, the contact zone between two fluids of different chemical or physical makeup.

Intermittent stream

A stream that flows only when it receives water from rainfall runoff or springs, or from some surface source such as melting snow.


Situated between or surrounded by mountains, mountain ranges, or mountainous regions.

Internal drainage

Surface drainage whereby the water does not reach the ocean, such as drainage toward the lowermost or central part of an interior basin or closed depression.


Alternately flooded and exposed by tides.

Intolerant organisms

Organisms that are not adaptable to human alterations to the environment and thus decline in numbers where alterations occur. See also Tolerant species.


An animal having no backbone or spinal column.


A positively or negatively charged atom or group of atoms.

Irrigation district

In the United States, a cooperative, self-governing public corporation set up as a subdivision of the state, with definite geographic boundaries, organized to obtain and distribute water for irrigation of lands within the district; created under authority of the State legislature with the consent of a designated fraction of the land owners or citizens and the taxing power.

Irrigation return flow

The part of irrigation applied to the surface that is not consumed by evapotranspiration or uptake by plants and that migrates to an aquifer or surface-water body.

Irrigation withdrawals

Withdrawals of water for application on land to assist in the growing of crops and pastures or to maintain recreational lands.



A type of topography that results from dissolution and collapse of carbonate rocks such as limestone, dolomite, and gypsum, and that is characterized by closed depressions or sinkholes, caves, and underground drainage.


A steep-sided hole or depression, commonly without surface drainage, formed by the melting of a large detached block of stagnant ice that had been buried in the glacial drift.

Kettle lake

A body of water occupying a kettle, as in a pitted outwash plain or in a kettle moraine.


Dutch term for stream or creek.



Pertaining to, produced by, or formed in a lake.

Lacustrine wetlands

Wetlands within a lake or reservoir greater than 20 acres or within a lake or reservoir less than 20 acres if the water is greater than 2 meters deep in the deepest part of the basin; ocean-derived salinity is less than 0.5 part per thousand.


A shallow stretch of seawater (or lakewater) near or communicating with the sea (or lake) and partly or completely separated from it by a low, narrow, elongate strip of land.

Land-use study

A network of existing shallow wells in an area having a relatively uniform land use. These studies are a subset of a NAWQA Study-Unit Survey and have the goal of relating the quality of shallow ground water to land use.

Latent heat

The amount of heat given up or absorbed when a substance changes from one state to another, such as from a liquid to a solid.

Lateral moraine

A low ridgelike moraine carried on, or deposited near, the side margin of a mountain glacier.


A liquid that has percolated through soil containing soluble substances and that contains certain amounts of these substances in solution.


The removal of materials in solution from soil or rock; also refers to movement of pesticides or nutrients from land surface to ground water.

Life zone

Major area of plant and animal life; region characterized by particular plants and animals and distinguished by temperature differences.


A sedimentary rock consisting chiefly of calcium carbonate, primarily in the form of the mineral calcite.


The deepwater zone (greater than 2 meters deep); a subsystem of the Lacustrine System of the U.S. Fish and Wildlife Service wetland classification system.


The shallow-water zone (less than 2 meters deep); a subsystem of the Lacustrine System of the U.S. Fish and Wildlife Service wetland classification system.


Material that is moved or carried by streams, reported as weight of material transported during a specified time period, such as tons per year.


A widespread, homogeneous, commonly nonstratified, porous, friable, slightly coherent, fine-grained blanket deposit of wind-blown and wind-deposited silt and fine sand.

Long-term monitoring

The collection of data over a period of years or decades to assess changes in selected hydrologic conditions.


Main stem

The principal trunk of a river or a stream.

Major ions

Constituents commonly present in water in concentrations exceeding 1.0 milligram per liter. Major cations are calcium, magnesium, sodium, and potassium; the major anions are sulfate, chloride, fluoride, nitrate, and those contributing to alkalinity (see alkaline), most generally assumed to be bicarbonate and carbonate.

Marine wetland

Wetlands that are exposed to waves and currents of the open ocean and to water having a salinity greater than 30 parts per thousand; present along the coastlines of the open ocean.


A water-saturated, poorly drained area, intermittently or permanently water covered, having aquatic and grasslike vegetation.


A stage in the evolutionary erosion of land areas in which the flat uplands have been widely dissected by deep river valleys.

Maturity (stream)

The stage in the development of a stream at which it has reached its maximum efficiency, when velocity is just sufficient to carry the sediment delivered to it by tributaries; characterized by a broad, open, flat-floored valley having a moderate gradient and gentle slope.

Maximum contaminant level

Maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are enforceable standards established by the U.S. Environmental Protection Agency.


The arithmatic average of a set of observations, unless otherwise specified.

Mean discharge

The arithmetic mean of individual daily mean discharges of a stream during a specific period, usually daily, monthly, or annually.

Mean high tide

The average altitude of all high tides recorded at a given place over a 19-year period.

Mean low tide

The average altitude of all low tides recorded at a given place over a 19-year period.


The middle or central value in a distribution of data ranked in order of magnitude. The median is also known as the 50th percentile.


Any plant growing where moisture and aeration conditions lie between the extremes of "wet" and "dry."


A substance produced in or by biological processes.

Metamorphic rocks

Rocks derived from preexisting rocks by mineralogical, chemical, or structural changes (essentially in a solid state) in response to marked changes in temperature, pressure, shearing stress, and chemical environment at depth in the Earth's crust.

Method detection limit

The minimum concentration of a substance that can be accurately identified and measured with current laboratory technologies.

Micrograms per liter

A unit expressing the concentration of constituents in solution as weight (micrograms) of solute per unit volume (liter) of water; equivalent to one part per billion in most streamwater and ground water. One thousand micrograms per liter equals one milligram per liter.


A small fly in the family Chironomidae. The larval (juvenile) life stages are aquatic.


A mass equal to 10-3 grams.

Milligrams per liter

A unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water; equivalent to one part per million in most streamwater and ground water.

Mineral soil

Soil composed predominantly of mineral rather than organic materials; less than 20 percent organic material.

Minimum reporting level

The smallest measured concentration of a constituent that may be reliably reported using a given analytical method. In many cases, the MRL is used when documentation for the method detection limit is not available.


Actions taken to avoid, reduce, or compensate for the effects of human-induced environmental damage.


Repeated observation, measurement, or sampling at a site, on a scheduled or event basis, for a particular purpose.

Monitoring well

A well designed for measuring water levels and testing ground-water quality.

Monocyclic aromatic hydrocarbons

Single-ring aromatic compounds. Constituents of lead-free gasoline; also used in the manufacture of monomers and plasticizers in polymers.


Of, pertaining to, or inhabiting cool upland slopes below the timber line; characterized by the dominance of evergreen trees.


A mound, ridge, or other distinct accumulation of unsorted, unstratified glacial drift, predominantly till, deposited chiefly by direct action of glacier ice.


The place where a stream discharges to a larger stream, a lake, or the sea.


Dark, finely divided, well-decomposed, organic matter forming a surface deposit in some poorly drained areas.


Large expanses of peatlands or bogs in subarctic zones.



National Water-Quality Assessment Program - The long term USGS program, begun in 1991, to assess the occurrence and distribution of water-quality conditions Nationwide.

National Academy of Sciences/National Academy of Engineering (NAS/NAE) recommended maximum concentration in water

Numerical guidelines recommended by two joint NAS/NAE committees for the protection of freshwater and marine aquatic life, respectively. These guidelines were based on results of aquatic toxicity studies, available in 1972, and were considered preliminary even at the time.

National Geodetic Vertical Datum of 1929

Geodetic datum derived from a general adjustment of first - order level nets of the United States and Canada; formerly called "Sea Level Datum of 1929."

Natural levee

A long, broad, low ridge built by a stream on its flood plain along one or both banks of its channel in time of flood.

Navigable water

In the context of the Clean Water Act, all surface water.


An ion consisting of nitrogen and oxygen (NO3-). Nitrate is a plant nutrient and is very mobile in soils.

Noncontact water recreation

Recreational activities, such as fishing or boating, that do not include direct contact with the water.

Nonpersistent emergent plants

Emergent plants whose leaves and stems break down at the end of the growing season from decay or by the physical forces of waves and ice; at certain seasons, there are no visible traces of the plants above the surface of the water.

Nonpoint source

A source (of any water-carried material) from a broad area, rather than from discrete points.

Nonpoint-source contaminant

A substance that pollutes or degrades water that comes from lawn or cropland runoff, the atmosphere, roadways, and other diffuse sources.

Nonpoint-source water pollution

Water contamination that originates from a broad area (such as leaching of agricultural chemicals from crop land) and enters the water resource diffusely over a large area.

Nonselective herbicide

Kills or significantly retards growth of most higher plant species.

Nuisance species

Undesirable plants and animals, commonly exotic species.


Any inorganic or organic compound needed to sustain plant life.


Occurrence and distribution assessment

A component of the USGS National Water Quality Assessment (NAWQA) Program that entails characterization of broad-scale spatial and temporal distributions of water-quality conditions in relation to major contaminant sources and background conditions for surface water and ground water.

Offstream use

Water withdrawn or diverted from a ground- or surface-water source for use.


Containing carbon, but possibly also containing hydrogen, oxygen, chlorine, nitrogen, and other elements.

Organic detritus

Any loose organic material in streams - such as leaves, bark, or twigs - removed and transported by mechanical means, such as disintegration or abrasion.

Organic soil

Soil that contains more than 20 percent organic matter in the upper 16 inches.

Organochlorine compound

Synthetic organic compounds containing chlorine. As generally used, term refers to compounds containing mostly or exclusively carbon, hydrogen, and chlorine. Examples include organochlorine insecticides, polychlorinated biphenyls, and some solvents containing chlorine.

Organochlorine insecticide

A class of organic insecticides containing a high percentage of chlorine. Includes dichlorodiphenylethanes (such as DDT), chlorinated cyclodienes (such as chlordane), and chlorinated benzenes (such as lindane). Most organochlorine insecticides were banned from use in the United States because of their carcinogenicity, tendency to bioaccumulate, and toxicity to wildlife.

Organonitrogen herbicides

A group of herbicides consisting of a nitrogen ring with associated functional groups and including such classes as triazines and acetanilides. Examples include atrazine, cyanazine, alachlor, and metolachlor.

Organophosphate insecticides

A class of insecticides derived from phosphoric acid. They tend to have high acute toxicity to vertebrates. Although readily metabolized by vertebrates, some metabolic products are more toxic than the parent compound.

Organophosphorus insecticides

Insecticides derived from phosphoric acid and generally the most toxic of all pesticides to vertebrate animals.


Pertaining to mountains, in regard to their location and distribution; said of the precipitation caused by the lifting of moisture-laden air over mountains.


Soil material washed down a hillside by rainwater and deposited upon more gently sloping land.

Overland flow

The flow of rainwater or snowmelt over the land surface toward stream channels.


A bow-shaped lake formed in an abandoned meander of a river.



A measure of the acidity (less than 7) or alkalinity (greater than 7) of a solution; a pH of 7 is considered neutral.


Study of hydrologic processes and events, using geological, botanical, and cultural evidence, that occurred before the beginning of the systematic collection of hydrologic data and observations.

Palustrine wetlands

Freshwater wetlands including open water bodies of less than 20 acres in which water is less than 2 meters deep; includes marshes, wet meadows, fens, playas, potholes, pocosins, bogs, swamps, and shallow ponds; most wetlands are in the Palustrine system.

Part per million

Unit of concentration equal to one milligram per kilogram or one milligram per liter.


Any living organism that causes disease.

Peak stage

Maximum height of a water surface above an established datum plane.


A highly organic soil, composed of partially decomposed vegetable matter.

Perched ground water

Unconfined ground water separated from an underlying main body of ground water by an unsaturated zone.


The movement, under hydrostatic pressure, of water through interstices of a rock or soil (except the movement through large openings such as caves).

Perennial stream

A stream that normally has water in its channel at all times.


Micro-organisms that coat rocks, plants, and other surfaces on lake bottoms.


Any frozen soil, subsoil, surficial deposit, or bedrock in arctic or subarctic regions where below-freezing temperatures have existed continuously from two to tens of thousands of years.


The capacity of a rock for transmitting a fluid; a measure of the relative ease with which a porous medium can transmit a liquid.


Any substance used to kill plant or animal pests; major categories of pesticides include herbicides and insecticides.


A class of organic compounds containing phenol (C6H5OH) and its derivatives. Used to make resins, weed killers, and as a solvent, disinfectant, and chemical intermediate. Some phenols occur naturally in the environment.


A nutrient essential for growth that can play a key role in stimulating aquatic growth in lakes and streams.


The synthesis of compounds with the aid of light.


A class of organic compounds containing phthalic acid esters [C6H4(COOR)2] and derivatives. Used as plasticizers in plastics. Also used in many other products (such as detergents, cosmetics) and industrial processes (such as defoaming agents in paper and paperboard manufacture, and dielectrics in capacitors).

Physiographic province

A region in which the landforms are distinctive and differ significantly from those of adjacent regions.


A description of the surface features of the Earth, with an emphasis on the origin of landforms.


One trillionth (10-12) of the amount of radioactivity represented by a curie (Ci). A curie is the amount of radioactivity that yields 3.7 x 1010 radioactive disintegrations per second (dps). A picocurie yields 2.22 disintegrations per minute (dpm) or 0.037 dps.

Pioneer plant

Herbaceous annual and perennial seedling plants that colonize bare areas as a first stage in secondary succession.


Erosion by percolating water in a layer of subsoil, resulting in caving and in the formation of narrow conduits, tunnels, or "pipes" through which soluble or granular soil material is removed.


A surficial mineral deposit formed by mechanical concentration of mineral particles from weathered debris.


Floating or weakly swimming organisms at the mercy of the waves and currents. Animals of the group are called zooplankton and the plants are called phytoplankton.


A dry, flat area at the lowest part of an undrained desert basin in which water accumulates and is quickly evaporated; underlain by stratified clay, silt, or sand and commonly by soluble salts; term used in Southwestern United States.

Playa lake

A shallow, temporary lake in an arid or semiarid region, covering or occupying a playa in the wet season but drying up in summer; temporary lake that upon evaporation leaves or forms a playa.


A local term along the Atlantic coastal plain, from Virginia south, for a shrub-scrub wetland located on a relatively flat terrain, commonly between streams.

Point source

Originating at any discrete source.

Point-source contaminant

Any substance that degrades water quality and originates from discrete locations such as discharge pipes, drainage ditches, wells, concentrated livestock operations, or floating craft.


Any substance that, when present in a hydrologic system at sufficient concentration, degrades water quality in ways that are or could become harmful to human and/or ecological health or that impair the use of water for recreation, agriculture, industry, commerce, or domestic purposes.

Polychlorinated biphenyls

A mixture of chlorinated derivatives of biphenyl, marketed under the trade name Aroclor with a number designating the chlorine content (such as Aroclor 1260). PCBs were used in transformers and capacitors for insulating purposes and in gas pipeline systems as a lubricant. Further sale for new use was banned by law in 1979.

Polycyclic aromatic hydrocarbon

A class of organic compounds with a fused-ring aromatic structure. PAHs result from incomplete combustion of organic carbon (including wood), municipal solid waste, and fossil fuels, as well as from natural or anthropogenic introduction of uncombusted coal and oil. PAHs include benzo(a)pyrene, fluoranthene, and pyrene.


A small part of a stream reach with little velocity, commonly with water deeper than surrounding areas.


A collection of individuals of one species or mixed species making up the residents of a prescribed area.


The ratio of the volume of voids in a rock or soil to the total volume.

Postemergence herbicide

Herbicide applied to foliage after the crop has sprouted to kill or significantly retard the growth of weeds.

Potable water

Water that is safe and palatable for human consumption.

Potential evapotranspiration

The amount of moisture which, if available, would be removed from a given land area by evapotranspiration; expressed in units of water depth.

Potentiometric surface

An imaginary surface that represents the total head in an aquifer. It represents the height above a datum plane at which the water level stands in tightly cased wells that penetrate the aquifer.

Prairie pothole

A shallow depression, generally containing wetlands, occurring in an outwash plain, a recessional moraine, or a till plain; usually the result of melted blocks of covered glacial ice; occur most commonly in the North-Central United States and in States west of the Great Lakes from Wisconsin to eastern Montana.


Any or all forms of water particles that fall from the atmosphere, such as rain, snow, hail, and sleet. The act or process of producing a solid phase within a liquid medium.

Preemergence herbicide

Herbicide applied to bare ground after planting the crop but prior to the crop sprouting above ground to kill or significantly retard the growth of weed seedlings.


The earliest condition of the quality of a water body; unaffected by human activities.

Public-supply withdrawals

Water withdrawn by public and private water suppliers for use within a general community. Water is used for a variety of purposes such as domestic, commercial, industrial, and public water use.


Quality assurance

Evaluation of quality-control data to allow quantitative determination of the quality of chemical data collected during a study. Techniques used to collect, process, and analyze water samples are evaluated.



A naturally occurring, colorless, odorless, radioactive gas formed by the disintegration of the element radium; damaging to human lungs when inhaled.

Rain shadow

A dry region on the lee side of a topographic obstacle, usually a mountain range, where rainfall is noticeably less than on the windward side.


A continuous part of a stream between two specified points.

Real-time data

Data collected by automated instrumentation and telemetered and analyzed quickly enough to influence a decision that affects the monitored system.

Recessional moraine

An end moraine built during a temporary but significant pause in the final retreat of a glacier.


The process involved in the absorption and addition of water to the zone of saturation.

Recharge area

An area within which water infiltrates the ground and reaches the zone of saturation.

Recurrence interval

The average interval of time within which the magnitude of a given event, such as a storm or flood, will be equaled or exceeded once.

Reference site

A NAWQA sampling site selected for its relatively undisturbed conditions.


The layer or mantle of fragmented and unconsolidated rock material, residual or transported, that nearly everywhere forms the surface of the land and overlies or covers the bedrock.


Artificial manipulation of the flow of a stream.

Relative abundance

The number of organisms of a particular kind present in a sample relative to the total number of organisms in the sample.

Retrospective analysis

Review and analysis of existing data in order to address NAWQA objectives, to the extent possible, and to aid in the design of NAWQA studies.

Return flow

That part of irrigation water that is not consumed by evapotranspiration and that returns to its source or another body of water.


A shallow part of the stream where water flows swiftly over completely or partially submerged obstructions to produce surface agitation.


Pertaining to or situated on the bank of a natural body of flowing water.

Riparian rights

A concept of water law under which authorization to use water in a stream is based on ownership of the land adjacent to the stream.

Riparian zone

Pertaining to or located on the bank of a body of water, especially a stream.

Riverine wetlands

Wetlands within river and stream channels; ocean-derived salinity is less than 0.5 part per thousand.


Any naturally formed, consolidated or unconsolidated material (but not soil) consisting of two or more minerals.


That part of precipitation or snowmelt that appears in streams or surface-water bodies.

Rural withdrawals

Water used in suburban or farm areas for domestic and livestock needs. The water generally is self-supplied and includes domestic use, drinking water for livestock, and other uses such as dairy sanitation, evaporation from stock-watering ponds, and cleaning and waste disposal.



An area where deposits of crystalline salt are formed, such as a salt flat; a body of saline water, such as a saline playa or salt marsh.

Saline water

Water that is considered unsuitable for human consumption or for irrigation because of its high content of dissolved solids; generally expressed as milligrams per liter (mg/L) of dissolved solids; seawater is generally considered to contain more than 35,000 mg/L of dissolved solids.

Saturated zone

A subsurface zone in which all the interstices or voids are filled with water under pressure greater than that of the atmosphere. See also Water table.

Sea level

Long-term average position of the sea surface. Sea level varies from place to place and with the time period for which the average is calculated. For the conterminous United States, sea level is most commonly referenced to the National Geodetic Vertical Datum of 1929.

Secondary maximum contaminant level

The maximum level of a contaminant or undesirable constituent in public water systems that, in the judgment of the U.S. Environmental Protection Agency (USEPA), is required to protect the public welfare. SMCLs are secondary (nonenforceable) drinking water regulations established by the USEPA for contaminants that may adversely affect the odor or appearance of such water.


Particles, derived from rocks or biological materials, that have been transported by a fluid or other natural process, suspended or settled in water.

Sediment guideline

Threshold concentration above which there is a high probability of adverse effects on aquatic life from sediment contamination, determined using modified U.S. Environmental Protection Agency USEPA (1996) procedures.

Sedimentary rocks

Rocks formed by the consolidation of loose sediment that has accumulated in layers.


The act or process of forming or accumulating sediment in layers; the process of deposition of sediment.


A small area where water percolates (see percolation) slowly to the land surface.


A sudden oscillation of the water in a moderate-size body of water, caused by wind.

Selective herbicide

A compound that kills or significantly retards growth of an unwanted plant species without significantly damaging desired plant species.

Semipermeable membrane device

A long strip of low-density, polyethylene tubing filled with a thin film of purified lipid such as triolein that simulates the exposure to and passive uptake of highly lipid-soluble organic compounds by biological membranes.

Semivolatile organic compound

Operationally defined as a group of synthetic organic compounds that are solvent-extractable and can be determined by gas chromatography/mass spectrometry. SVOCs include phenols, phthalates, and Polycyclic aromatic hydrocarbons (PAHs).


A fine-grained sedimentary rock formed by the consolidation of clay, silt, or mud.


A term applied to a shallow place or area in a body of water; a shoal.


A relatively shallow place in a stream, lake, or sea.

Short-wave trough

A wave of low atmospheric pressure in the form of a trough that has a wave length of 600 to 1,500 miles and moves progressively through the lower troposphere in the same direction as that of the prevailing current of air motion.


Land covered predominantly with shrubs.

Sideslope gradient

The representative change in elevation in a given horizontal distance (usually about 300 yards) perpendicular to a stream; the valley slope along a line perpendicular to the stream (near a water-quality or biological sampling point).

Siliciclastic rocks

Rocks such as shale and sandstone that are formed by the compaction and cementation of quartz-rich mineral grains.


The deposition or accumulation of silt (or small-grained material) in a body of water.


An indurated silt having the texture and composition of shale but lacking its fine lamination.


The cultivation of forest trees.


A depression in an area underlain by limestone. Its drainage is subterranean.


The ratio of the channel length between two points on a channel to the straight-line distance between the same two points; a measure of meandering.


Numerical measure of the lack of symmetry of an asymmetrical frequency distribution.


A small marshy tract lying in a swale or other local shallow, undrained depression; a sluggish creek or channel in a wetland.


The layer of material at the land surface that supports plant growth.

Soil horizon

A layer of soil that is distinguishable from adjacent layers by characteristic physical and chemical properties.

Soil moisture

Water occurring in the pore spaces between the soil particles in the unsaturated zone from which water is discharged by the transpiration of plants or by evaporation from the soil.

Sole-source aquifer

As defined by the U.S. Environmental Protection Agency, an aquifer that supplies 50 percent or more of the drinking water of an area.

Solid-phase extraction

A procedure to isolate specific organic compounds onto a bonded silica extraction column.


Formed when a solid, gas, or another liquid in contact with a liquid becomes dispersed homogeneously throughout the liquid. The substance, called a solute, is said to dissolve. The liquid is called the solvent.


To take up and hold either by absorption or adsorption.


General term for the interaction (binding or association) of a solute ion or molecule with a solid.

Source rocks

The rocks from which fragments and other detached pieces have been derived to form a different rock.


Populations of organisms that may interbreed and produce fertile offspring having similar structure, habits, and functions.

Species diversity

An ecological concept that incorporates both the number of species in a particular sampling area and the evenness with which individuals are distributed among the various species.

Species richness

The number of species (taxa) present in a defined area or sampling unit.

Specific capacity

The yield of a well per unit of drawdown.

Specific conductance

A measure of the ability of a liquid to conduct an electrical current.

Specific yield

The ratio of the volume of water that will drain under the influence of gravity to the volume of saturated rock.


A small point or low tongue or narrow embankment of land having one end attached to the mainland and the other terminating in open water.

Split sample

A sample prepared by dividing it into two or more equal volumes, so that each volume is considered a separate sample but representative of the entire sample.


Overburden or other waste material removed in mining, quarrying, dredging, or excavating.


Place where a concentrated discharge of ground water flows at the ground surface.


Height of the water surface above an established datum plane, such as in a river above a predetermined point that may (or may not) be at the channel floor.

Standard deviation

Statistical measure of the dispersion or scatter of a series of values. It is the square root of the variance, which is calculated as the sum of the squares of the deviations from the arithmetic mean, divided by the number of values in the series minus 1.

State climate division

Geographic area in a State based primarily on crop-reporting districts. States can have 2 to 10 climate divisions.


A branch of mathematics dealing with the collection, analysis, interpretation, and presentation of masses of numerical data.

Storm surge

An abnormal and sudden rise of the sea along a shore as a result of the winds of a storm.


Subdivision of the environmental framework. NAWQA Study Units are divided into subareas that exhibit reasonably homogeneous environmental conditions, as determined by both natural and human influences.

Stream mile

A distance of 1 mile along a line connecting the midpoints of the channel of a stream.

Stream reach

A continuous part of a stream between two specified points.

Stream-aquifer interactions

Relations of water flow and chemistry between streams and aquifers that are hydraulically connected.


The discharge of water in a natural channel.


A line on a map that is parallel to the direction of fluid flow and shows flow patterns.

Study Unit

A major hydrologic system of the United States in which NAWQA studies are focused. Study Units are geographically defined by a combination of ground- and surface-water features and generally encompass more than 4,000 square miles of land area.

Study-Unit Survey

Broad assessment of the water-quality conditions of the major aquifer systems of each NAWQA Study Unit. The Study-Unit Survey relies primarily on sampling existing wells and, wherever possible, on data collected by other agencies and programs. Typically, 20 to 30 wells are sampled in each of three to five aquifer subunits.

Submersed plant

A plant that lies entirely beneath the water surface, except for flowering parts in some species.


The gradual downward settling or sinking of the Earth's surface with little or no horizontal motion.


The surface beneath a wetland, lake, or stream in which organisms grow or to which organisms are attached.

Substrate size

The diameter of streambed particles such as clay, silt, sand, gravel, cobble and boulders.

Subsurface drain

A shallow drain installed in an irrigated field to intercept the rising ground-water level and maintain the water table at an acceptable depth below the land surface.


Continuously submerged; an area affected by ocean tides.

Subtropical anticyclone

A semipermanent anticyclone located, on the average, over oceans near 30 o N. and 30 o S. latitude.

Surface runoff

Runoff that travels over the land surface to the nearest stream channel.

Surface water

An open body of water such as a lake, river, or stream.


Sampling of a representative number of sites during a given hydrologic condition.


(as used in tables of chemical analyses)The amount (concentration) of undissolved material in a water-sediment mixture. Most commonly refer to that material retained on a 0.45- micrometer filter.

Suspended sediment

Sediment that is transported in suspension by a stream.

Suspended solids

Different from suspended sediment only in the way that the sample is collected and analyzed.

Suspended-sediment concentration

The velocity-weighted concentration of suspended sediment in the sampled zone (from the water surface to a point approximately 0.3 foot above the bed); expressed as milligrams of dry sediment per liter of water-sediment mixture (mg/L).


A slight depression, sometimes filled with water, in the midst of generally level land.


An area intermittently or permanently covered with water, and having trees and shrubs.

Synoptic sites

Sites sampled during a short-term investigation of specific water-quality conditions during selected seasonal or hydrologic conditions, to provide improved spatial resolution for critical water-quality conditions.



Rock that remains after processing ore to remove the valuable minerals.


A relatively small and deep, steep-sided lake or pool occupying an ice-gouged basin amid glaciated mountains.


Any identifiable group of taxonomically related organisms.

Tectonic activity

Movement of the Earth's crust resulting in the formation of ocean basins, continents, plateaus, and mountain ranges.

Terminal moraine

The end moraine extending across a glacial plain or valley as an arcuate or crescent ridge that marks the farthest advance or maximum extent of a glacier.


Area or surface over which a particular rock type or group of rock types is prevalent.


Pertaining to, consisting of, or representing the Earth.

Tertiary-treated sewage

The third phase of treating sewage that removes nitrogen and phosphorus before it is discharged.

Thermal loading

Amount of waste heat discharged to a water body.

Thermoelectric power

Electrical power generated by use of fossil-fuel (coal, oil, or natural gas), geothermal, or nuclear energy.


An irregular land surface formed in a permafrost region by melting ground ice and a subsequent settling of the ground.

Tidal flat

An extensive, nearly horizontal, tract of land that is alternately covered and uncovered by the tide and consists of unconsolidated sediment.


The rhythmic, alternate rise and fall of the surface (or water level) of the ocean, and connected bodies of water, occurring twice a day over most of the Earth, resulting from the gravitational attraction of the Moon, and to a lesser degree, the Sun.

Tier 1 sediment guideline

Threshold concentration above which there is a high probability of adverse effects on aquatic life from sediment contamination, determined using modified U.S. Environmental Protection Agency USEPA (1996) procedures.

Tile drain

A buried perforated pipe designed to remove excess water from soils.


Predominantly unsorted and unstratified drift, deposited directly by and underneath a glacier without subsequent reworking by meltwater, and consisting of a heterogeneous mixture of clay, silt, sand, gravel, and boulders.


A pocket of water developed below a waterfall; a term used in the Southwestern United States; used loosely to mean a temporary pool.

Tissue study

The assessment of concentrations and distributions of trace elements and certain organic contaminants in tissues of aquatic organisms.

Tolerant species

Those species that are adaptable to (tolerant of) human alterations to the environment and often increase in number when alterations occur.


The general configuration of a land surface or any part of the Earth's surface, including its relief and the position of its natural and man-made features.

Total DDT

The sum of DDT and its metabolites (breakdown products), including DDD and DDE.

Total concentration

Refers to the concentration of a constituent regardless of its form (dissolved or bound) in a sample.

Total head

The height above a datum plane of a column of water. In a ground-water system, it is composed of elevation head and pressure head.

Trace element

A chemical element that is present in minute quantities in a substance.


A stable, easily detected substance or a radioisotope added to a material to follow the location of the substance in the environment or to detect any physical or chemical changes that it undergoes.

Trade winds

A system of easterly winds that dominate most of the tropics. A major component of the general circulation of the atmosphere.


The rate at which water of the prevailing kinematic viscosity is transmitted through a unit width of an aquifer under a unit hydraulic gradient. It equals the hydraulic conductivity multiplied by the aquifer thickness.


The process by which water passes through living organisms, primarily plants, into the atmosphere.

Triazine herbicide

A class of herbicides containing a symmetrical triazine ring (a nitrogen-heterocyclic ring composed of three nitrogens and three carbons in an alternating sequence). Examples include atrazine, propazine, and simazine.


A river or stream flowing into a larger river, stream or lake.


A radioactive form of hydrogen with atoms of three times the mass of ordinary hydrogen; can be used to determine the age of water.

Tropical cyclone

A cyclone that originates over the tropical oceans. Tropical cyclones are classified according to their intensity and windspeed and, when fully mature, are characterized by extremely high-speed winds and torrential rains. In the United States, tropical cyclones that have windspeeds greater than 40 miles per hour are classified as tropical storms, and tropical cyclones that have windspeeds of 74 miles per hour or more are classified as hurricanes.


Lowest 6 to 12 miles of the atmosphere, characterized by a general decrease in temperature with height, appreciable water content, and active weather processes.


A vast, nearly level, treeless plain of the arctic and subarctic regions. It usually has a marshy surface which supports mosses, lichens, and low shrubs, underlain by mucky soils and permafrost.


The state, condition, or quality of opaqueness or reduced clarity of a fluid due to the presence of suspended matter.



The neutral form of an ionizable compound (such as an acid or a base).

Un-ionized ammonia

The neutral form of ammonia-nitrogen in water, usually occurring as NH4OH. Un-ionized ammonia is the principal form of ammonia that is toxic to aquatic life. The relative proportion of un-ionized to ionized ammonia (NH4+) is controlled by water temperature and pH. At temperatures and pH values typical of most natural waters, the ionized form is dominant.

Unconfined aquifer

An aquifer whose upper surface is a water table free to fluctuate under atmospheric pressure.

Unconsolidated deposit

Deposit of loosely bound sediment that typically fills topographically low areas.

Underground water

Subsurface water in the unsaturated and saturated zones.


A foliage layer lying beneath and shaded by the main canopy of a forest.

Unsaturated zone

A subsurface zone above the water table in which the pore spaces may contain a combination of air and water.


Of or pertaining to the place(s) from which ground water originated or traveled through before reaching a given point in an aquifer.


A general term for nonwetland; elevated land above low areas along streams or between hills; any elevated region from which rivers gather drainage.


A heavy silvery-white metallic element, highly radioactive and easily oxidized. Of the 14 known isotopes of uranium, U238 is the most abundant in nature.

Urban site

A site that has greater than 50 percent urbanized and less than 25 percent agricultural area.


Vascular plant

A plant composed of or provided with vessels or ducts that convey water or sap. A fern is an example of this type of plant.

Vernal pool

A small lake or pond that is filled with water for only a short time during the spring.

Volatile organic compounds

Organic chemicals that have a high vapor pressure relative to their water solubility. VOCs include components of gasoline, fuel oils, and lubricants, as well as organic solvents, fumigants, some inert ingredients in pesticides, and some by-products of chlorine disinfection.



A waterway used to drain excess irrigation water dumped from the irrigation delivery system.

Water budget

An accounting of the inflow to, outflow from, and storage changes of water in a hydrologic unit.

Water column

An imaginary column extending through a water body from its floor to its surface.

Water column studies

Investigations of physical and chemical characteristics of surface water, which include suspended sediment, dissolved solids, major ions, and metals, nutrients, organic carbon, and dissolved pesticides, in relation to hydrologic conditions, sources, and transport.

Water content of snow

Amount of liquid water in the snow at the time of observation. Water equivalent of snow.

Water demand

Water requirements for a particular purpose, such as irrigation, power, municipal supply, plant transpiration, or storage.

Water exports

Artificial transfer (by pipes or canals) of freshwater from one region or subregion to another.

Water gap

A deep, narrow pass in a mountain ridge through which a stream flows.

Water imports

Artificial transfer (by pipes or canals) of freshwater to one region or subregion from another.

Water rights

Legal rights to the use of water.

Water table

The top water surface of an unconfined aquifer at atmospheric pressure.

Water year

A continuous 12-month period selected to present data relative to hydrologic or meteorological phenomena during which a complete annual hydrologic cycle normally occurs. The water year used by the U.S. Geological Survey runs from October 1 through September 30, and is designated by the year in which it ends.

Water-quality criteria

Specific levels of water quality which, if reached, are expected to render a body of water unsuitable for its designated use. Commonly refers to criteria established by the U.S. Environmental Protection Agency. Water-quality criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes.

Water-quality guidelines

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

Water-quality standards

State-adopted and U.S. Environmental Protection Agency-approved ambient standards for water bodies. Standards include the use of the water body and the water-quality criteria that must be met to protect the designated use or uses.

Water-resources region

Natural drainage basin or hydrologic area that contains either the drainage area of a major river or the combined areas of a series of rivers. In the United States, there are 21 regions of which 18 are in the conterminous United States, and one each in Alaska, Hawaii, and the Caribbean.

Water-resources subregion

Subdivision of a water-resources region. The 21 water-resources regions of the United States are subdivided into 222 subregions. Each subregion includes that area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage area.


State of the atmosphere at any particular time and place.


Process whereby earthy or rocky materials are changed in color, texture, composition, or form (with little or no transportation) by exposure to atmospheric agents.

Weighted mean

A value obtained by multiplying each of a series of values by its assigned weight and dividing the sum of these products by the sum of the weights. In the ordinary arithmetic mean, each value is assigned a weight of 1.

Wetland function

A process or series of processes that take place within a wetland that are beneficial to the wetland itself, the surrounding ecosystems, and people.


Ecosystems whose soil is saturated for long periods seasonally or continuously, including marshes, swamps, and ephemeral ponds.

Willow carr

A pool, or wetland dominated by willow trees or shrubs.


Water removed from the ground or diverted from a surface-water source for use. Also refers to the use itself; for example, public-supply withdrawals or public-supply use.



A plant adapted for growth under dry conditions.



The mass of material or constituent transported by a river in a specified period of time divided by the drainage area of the river basin.