National Water-Quality Assessment (NAWQA) Program

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Effects of Urbanization on Stream Ecosystems

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Glossary of Terms

Algae - 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.

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

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

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

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

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

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

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

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

Biota - All living organisms of an area.

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.

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.

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.

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

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

Community composition scores (CCS) - Information about the differences and similarities in the species composition of a community across all sites in a study area. The difference between any two CCS values indicates the extent that the biological community is different between the two sites, which can then be related to the degree of urban development at the two sites.

Concentration - 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.

Contamination - 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.

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

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

Detect - 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.

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

Direct runoff - The runoff entering stream channels promptly after rainfall or snowmelt.

Discharge - 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.

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.

Diversion - 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.

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.

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

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.

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

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

Effluent - 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.

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.

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.

Equal-width increment (EWI) 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.

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

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

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.

Fertilizer - 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.

Fish community - In ecology, the species of fish that interact in a common area.

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

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

Fluvial - 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.

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. See also Water-quality criteria.

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

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

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

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

Headwaters - 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.

Herbicide - 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.

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

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

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

Hydrophobic - Not capable of uniting with or absorbing water.

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

Impermeability - The incapacity of a rock to transmit a fluid.

Impervious - Impermeable. See Impermeability.

Index of Biotic Integrity (IBI) - An aggregated number, or index, based on several attributes or metrics of a fish community that provides an assessment of biological conditions.

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.

Inorganic - Containing no carbon; matter other than plant or animal.

Insecticide - 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.

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.

Intolerant organisms - Organisms that are not adaptable to human alterations to the environment and thus decline in numbers where alterations occur.

Invertebrate - An animal having no backbone or spinal column. See also Benthic invertebrate.

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

Maximum contaminant level (MCL) - 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.

Mean - 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.

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

Metabolite - A substance produced in or by biological processes.

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

Micrograms per liter (µg/L) - 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.

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

Milligram (mg) - A mass equal to 10-3 grams.

Milligrams per liter (mg/L) - 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.

Minimum reporting level (MRL) - 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.

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

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

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

Nutrient - Any inorganic or organic compound needed to sustain plant life.

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

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.

Organochlorine pesticide - See Organochlorine insecticide.

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.

Part per million (ppm) - Unit of concentration equal to one milligram per kilogram or one milligram per liter.

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

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

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

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

Photosynthesis - The synthesis of compounds with the aid of light.

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

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

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.

Pollutant - 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 (PCBs) - 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 (PAH) - 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.

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

Population – In ecology, a collection of individuals of one species or mixed species making up the residents of a prescribed area.

Precipitation - 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.

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.

Reach - A continuous part of a stream between two specified points.

Regulation (of a stream) - 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.

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

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

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

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

Secondary maximum contaminant level (SMCL) - 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.

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

Semipermeable membrane device (SPMD) - 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 (SVOC) - 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).

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

Sinuosity - 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.

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

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

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

Stage - 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.

Stream order - A ranking of the relative sizes of streams within a watershed based on the nature of their tributaries. The smallest unbranched tributary is called first order, the stream receiving the tributary is called second order, and so on.

Stream reach - A continuous part of a stream between two specified points.

Streamflow - The discharge of water in a natural channel.

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.

Substrate - 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.

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

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

Suspended sediment - Sediment that is transported in suspension by a stream.

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).

Taxa richness - See Species richness.

Taxon (plural taxa) - Any identifiable group of taxonomically related organisms.

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

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.

Triazine pesticide - See Triazine herbicide.

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

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-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.

Watershed - See drainage basin.

For additional definitions see Water Basics Glossary


Carr, J.E., Chase, E.B., Paulson, R.W., Moody, D.W., compilers, 1990, National Water Summary 1987--Hydrologic events and water supply and use: U.S. Geological Survey Water-Supply Paper 2350, p. 546-549.

Fretwell, J.D., Williams, J.S., Redman, P.J., compilers, 1996, National Water Summary--Wetland Resources: U.S. Geological Survey Water-Supply Paper 2425, p. 425-431.

Moody, D.W., Carr, J.E., Chase, E.B., Paulson, R.W., compilers, 1988, National Water Summary 1986--Hydrologic events and ground-water quality: U.S. Geological Survey Water-Supply Paper 2325, p. 548-552.

Moody, D.W., Chase, E.B., Aronson, D.A., compilers, 1986, National Water Summary 1985 --Hydrologic events and surface-water resources: U.S. Geological Survey Water-Supply Paper 2300, p. 500-502.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National Water Summary 1988-89--Hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, p. 584-588.

Paulson, R.W., Chase, E.B., Williams, J.S., and Moody, D.W., compilers, 1993, National Water Summary 1990-91--Hydrologic events and stream water quality: U.S. Geological Survey Water-Supply Paper 2400, p. 578-585.

Paulson, R.W., Chase, E.B., Williams, J.S., and Moody, D.W., compilers, 1985, National Water Summary 1984--Hydrologic events, selected water-quality trends, and ground-water resources: U.S. Geological Survey Water-Supply Paper 2275, p. 460-464.

U.S. Environmental Protection Agency, 1996, The National Sediment Quality Survey: A report to Congress on the extent and severity of sediment contamination in surface waters of the United States: U.S. Environmental Protection Agency, Office of Science and Technology, Draft Report EPA 823-D-96-002.

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