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Glossary & Acronyms

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Adaptive management
Approach where source controls are initiated while additional monitoring data are collected to provide a basis for future review and revision of the TMDL (as well as management activities).
The raising of the bed of a watercourse by the deposition of sediment.
That portion of a receiving water's loading capacity that is attributed to one of its existing or future pollution sources (nonpoint or point) or to natural background sources. (A wasteload allocation [WLA] is that portion of the loading capacity allocated to an existing or future point source, and a load allocation [LA] is that portion allocated to an existing or future nonpoint source or to natural background source. Load allocations are best estimates of the loading, which can range from reasonably accurate estimates to gross allotments, depending on the availability of data and appropriate techniques for predicting loading.)
Sediment deposited by flowing water, such as in a riverbed, floodplain, or delta. Ambient water quality. Natural concentration of water quality constituents prior to mixing of either point or nonpoint source load of contaminants. Reference ambient concentration is used to indicate the concentration of a chemical that will not cause adverse impact to human health.
Ambient water quality
Natural concentration of water quality constituents prior to mixing of either point or nonpoint source load of contaminants. Reference ambient concentration is used to indicate the concentration of a chemical that will not cause adverse impact to human health.
Pertains to the [environmental] influence of human activities.
Anti-degradation policies
Policies that are part of each state's water quality standards. These policies are designed to protect water quality and provide a method of assessing activities that may impact the integrity of waterbodies.
Aquatic buffers
Streamside vegetation that filters stormwater and protects stream banks.
Aquatic classification system
Assigns a classification to a waterbody reflecting the water quality and the biological health (integrity). Classification is determined through use of biological indices (see IBI). Examples of classifications include oligosaprobic (cleanest water quality) and polysaprobic (highly polluted water).
Aquatic corridor
The area where land and water meet. This can include floodplains, stream channels, springs and seeps, small estuarine coves, littoral areas, stream crossings, shorelines, riparian forest, caves, and sinkholes.
Aquatic ecosystem
Complex of biotic and abiotic components of natural waters. The aquatic ecosystem is an ecological unit that includes the physical characteristics (such as flow or velocity and depth), the biological community of the water column and benthos, and the chemical characteristics such as dissolved solids, dissolved oxygen, and nutrients. Both living and nonliving components of the aquatic ecosystem interact and influence the properties and status of each component.
Aquatic life use
A use designation in State/Tribal water quality standards that generally provides for survival and reproduction of desirable fish, shellfish, and other aquatic organisms; classifications specified in state water quality standards relating to the level of protection afforded to the resident biological community.
Assimilative capacity
The amount of contaminant load that can be discharged to a specific waterbody without exceeding water quality standards or criteria. Assimilative capacity is used to define the ability of a waterbody to naturally absorb and use a discharged substance without impairing water quality or harming aquatic life.
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Background levels
Levels representing the chemical, physical, and biological conditions that would result from natural geomorphological processes such as weathering or dissolution.
Base level
Lowest point to which a stream may erode its channel; the ultimate base level is sea level; temporary or local base levels are defined by rock, hardpan, or other strata that resist downcutting and force erosional processes laterally.
BASINS (Better Assessment Science Integrating Point and Nonpoint Sources)
A computer-run tool that contains an assessment and planning component that allows users to organize and display geographic information for selected watersheds. It also contains a modeling component to examine impacts of pollutant loadings from point and nonpoint sources and to characterize the overall condition of specific watersheds.
Bedload sediment
Portion of sediment load transported downstream by sliding, rolling, &/or bouncing along the channel bottom. Generally consists of particles >1 mm.
Refers to material, especially sediment, at the bottom of an aquatic ecosystem. It can be used to describe the organisms that live on, or in, the bottom of a waterbody.
Benthic organisms
Organisms living in, or on, bottom substrates in aquatic ecosystems.
Best management practices (BMPs)
Methods, measures, or practices that are determined to be reasonable and cost-effective means for a land owner to meet certain, generally nonpoint source, pollution control needs. BMPs include structural and nonstructural controls and operation and maintenance procedures.
Biological assessment; the evaluation of an ecosystem using integrated assessments of habitat and biological communities in comparison to empirically defined reference conditions. Biochemical oxygen demand (BOD)
The amount of oxygen per unit volume of water required to bacterially or chemically oxidize (stabilize) the oxidizable matter in water. Biochemical oxygen demand measurements are usually conducted over specific time intervals (5, 10, 20, 30 days). The term BOD generally refers to a standard 5-day BOD test.
Biochemical oxygen demand (BOD)
The amount of oxygen per unit volume of water required to bacterially or chemically oxidize (stabilize) the oxidizable matter in water. Biochemical oxygen demand measurements are usually conducted over specific time intervals (5, 10, 20, 30 days). The term BOD generally refers to a standard 5-day BOD test.
Biological criteria
Also known as biocriteria, biological criteria are narrative expressions or numeric values of the biological characteristics of aquatic communities based on appropriate reference conditions. Biological criteria serve as an index of aquatic community health.
The amount, or weight, of a species, or group of biological organisms, within a specific volume or area of an ecosystem.
Boundary conditions
Values or functions representing the state of a system at its boundary limits.
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Pertaining to or containing calcium carbonate.
The process of adjusting model parameters within physically defensible ranges until the resulting predictions give a best possible good fit to observed data.
Pertaining to or containing carbon derived from plant and animal residues.
Cation exchange capacity
The sum total of exchangeable cations that a soil can adsorb. Expressed in centimoles per kilogram of soil (or of other adsorbing material such as clay.)
A natural stream that conveys water; a ditch or channel excavated for the flow of water.
Channel improvement
The improvement of the flow characteristics of a channel by clearing, excavation, realignment, lining, or other means in order to increase its capacity. Sometimes used to connote channel stabilization.
Channel morphology
The change in a stream channel's width or the shape of the stream banks. Increased erosion often causes a stream channel to widen and to deepen. Additional aspects of channel morphology include height, angle, and extent of bank erosion, substrate embeddedness, sediment deposition, and substrate.
Channel stabilization
Erosion prevention and stabilization of velocity distribution in a channel using jetties, drops, revetments, vegetation, and other measures.
An atom of chlorine in solution; an ion bearing a single negative charge.
Clean sediment
Sediment that is not contaminated by chemical substances. Pollution caused by clean sediment refers to the quantity of sediment, as opposed to the presence of pollutant-contaminated sediment.
Clean Water Act (CWA)
The Clean Water Act (formerly referred to as the Federal Water Pollution Control Act or Federal Water Pollution Control Act Amendments of 1972), Public Law 92-500, as amended by Public Law 96-483 and Public Law 97-117, 33 U.S.C. 1251 et seq. The Clean Water Act (CWA) contains a number of provisions to restore and maintain the quality of the nation's water resources. One of these provisions is section 303(d), which establishes the TMDL program.
Coastal zone
Lands and waters adjacent to the coast that exert an influence on the uses of the sea and its ecology, or whose uses and ecology are affected by the sea.
Soil and rock debris on a hillslope that has been transported from its original location.
Amount of a substance or material in a given unit volume of solution; usually measured in milligrams per liter (mg/L) or parts per million (ppm).
Concentration-based limit
A limit based on the relative strength of a pollutant in a wastestream, usually expressed in milligrams per liter (mg/L).
Contaminated sediments
Deposited or accumulated sediments, typically on the bottom of a waterbody, that contain contaminants. These may or may not be toxic as revealed by a whole sediment toxicity test, or as predicted by equilibrium partitioning.
The act of polluting or making impure; any indication of chemical, sediment, or biological impurities.
Continuous discharge
A discharge that occurs without interruption throughout the operating hours of a facility, except for infrequent shutdowns for maintenance, process changes, or other similar activities.
Conventional pollutants
As specified under the Clean Water Act, conventional contaminants include suspended solids, coliform bacteria, high biochemical oxygen demand, pH, and oil and grease.
Cost-share program
A program that allocates project funds to pay a percentage of the cost of constructing or implementing a best management practice. The remainder of the costs are paid by the producer.
   (1) Under section 304(a) of the Clean Water Act, EPA publishes scientific information regarding concentrations of specific chemicals or levels of parameters in water that protect aquatic life and human health.
   (2)Levels of individual pollutants, or water quality characteristics, or descriptions of conditions of a water body, adopted into State water quality standards that, if met, will generally protect the designated use of the water. In many cases, States make use of the criteria developed by EPA under definition #1 above.
Critical condition
The critical condition can be thought of as the "worst case" scenario of environmental conditions in the waterbody in which the loading expressed in the TMDL for the pollutant of concern will continue to meet water quality standards. Critical conditions are the combination of environmental factors (e.g., flow, temperature, etc.) that results in attaining and maintaining the water quality criterion and has an acceptably low frequency of occurrence.
Cross-sectional area
Wet area of a waterbody normal to the longitudinal component of the flow.
See protozoa.
Cumulative impacts
Describes situations when the effects of an action are added to or interact with other effects in a particular place and within a particular time. A multi-purpose practice used for the removal of sediment that accumulates at the bottom of water bodies.
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The gradual decrease in the amount of a given substance in a given system due to various sink processes including chemical and biological transformation, dissipation to other environmental media, or deposition into storage areas.
Metabolic breakdown of organic materials; the formation of by-products of decomposition releases energy and simple organic and inorganic compounds. (See also, Respiration.)
Design stream flow
The stream flow used to conduct steady-state wasteload allocation modeling.
Designated uses
Those uses specified in State/Tribal water quality standards for each water body or segment whether or not they are being attained. Sometimes referred to as Beneficial Uses, i.e., desirable uses that water quality should support. Examples are drinking water supply, primary contact recreation (such as swimming), and aquatic life support.
Deterministic model
A model that does not include built-in variability: same input will always equal the same output.
Any loose material produced directly from disintegration processes. Organic detritus consists of material resulting from the decomposition of dead organic remains.
Production of sediment fluxes as a result of the flux of particulate organic carbon in the sediment and its decomposition. The diagenesis reaction can be thought of as producing oxygen equivalents released by various reduced species.
Diel ("die´-el")
Involving a 24-hour period.
The addition of some quantity of less concentrated liquid (water) that results in a decrease in the original concentration.
Direct runoff
Water that flows over the ground surface or through the ground directly into streams, rivers, and lakes.
Flow of surface water in a stream or canal or the outflow of groundwater from a flowing artesian well, ditch, or spring. Can also apply to discharge of liquid effluent from a facility or to chemical emissions into the air through designated venting mechanisms.
Discharge monitoring report (DMR)
Report of effluent characteristics submitted by a municipal or industrial facility that has been granted an NPDES discharge permit.
Discharge permits (NPDES)
A permit issued by the U.S. EPA or a State regulatory agency that sets specific limits on the type and amount of pollutants that a municipality or industry can discharge to a receiving water; it also includes a compliance schedule for achieving those limits. It is called the NPDES because the permit process was established under the National Pollutant Discharge Elimination System, under provisions of the Federal Clean Water Act.
The spreading of chemical or biological constituents, including pollutants, in various directions from a point source, at varying velocities depending on the differential in-stream flow characteristics.
Dissolved oxygen (DO)
The amount of oxygen that is dissolved in water. This term also refers to a measure of the amount of oxygen available for biochemical activity in a waterbody, and is an indicator of the quality of that water.
Domestic wastewater
Also called sanitary wastewater, consists of wastewater discharged from residences and from commercial, institutional, and similar facilities.
Drainage basin
A part of a land area enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into a receiving water. Also referred to as a watershed, river basin, or hydrologic unit.
Dry ravel
Sloughing of sediment due to loss of cohesion in surface materials.
Dynamic model
A mathematical formulation describing and simulating the physical behavior of a system or a process and its temporal variability.
Dynamic simulation
Modeling of the behavior of physical, chemical, and/or biological phenomena and their variation over time.
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Ecological function
Potentially impacted by changes in channel morphology, stream hydrology, water quality, and habitat structure. Ecological function can be measured by fish diversity, macroinvertebrate diversity, biological integrity, EPA Rapid Bioassessment Protocol, fish barriers, and the leaf pack processing rate.
A physical region that is defined by its ecology, which includes meteorological factors, elevation, plant and animal speciation, landscape position, and soils.
An interactive system that includes the organisms of a natural community association together with their abiotic physical, chemical, and geochemical environment.
Municipal sewage or industrial liquid waste (untreated, partially treated, or completely treated) that flows out of a treatment plant, septic system, pipe, etc.
Effluent guidelines
Technical EPA documents that set effluent limitations for given industries and pollutants.
Effluent limitation
Restrictions established by a state or EPA on quantities, rates, and concentrations in pollutant discharges.
Effluent plume
Delineates the extent of contamination in a given medium as a result of a distribution of effluent discharges (or spills). Usually shows the concentration gradient within the delineated areas or plume of flow of contaminants.
The degree to which fine sediments (e.g., clays, silts) fill the spaces (interstices) between rocks, cobbles, and gravel on the bottom of a stream or river.
Empirical model
Use of statistical techniques to discern patterns or relationships underlying observed or measured data for large sample sets. Does not account for physical dynamics of waterbodies.
An endpoint (or indicator/target) is a characteristic of an ecosystem that may be affected by exposure to a stressor. Assessment endpoints and measurement endpoints are two distinct types of endpoints commonly used by resource managers. An assessment endpoint is the formal expression of a valued environmental characteristic and should have societal relevance (an indicator). A measurement endpoint is the expression of an observed or measured response to a stress or disturbance. It is a measurable environmental characteristic that is related to the valued environmental characteristic chosen as the assessment endpoint. The numeric criteria that are part of traditional water quality standards are good examples of measurement endpoints (targets).
In the context of restoration ecology, any improvement of a structural or functional attribute.
Environmental monitoring and assessment program (EMAP)
A USEPA program to monitor and assess the ecological health of major ecosystems, including surface waters, forests, near-coastal waters, wetlands, agricultural lands, arid lands, and the Great Lakes, in an integrated, systematic manner. Although EMAP has been curtailed somewhat during recent years, the program is designed to operate at regional and national scales, for decades, and to evaluate the extent and condition of entire ecological resources by using a common sampling framework to sample approximately 12,500 locations in the conterminous United States.
Brackish-water areas influenced by the tides where the mouth of a river meets the sea.
Estuarine number
A nondimensional parameter accounting for decay, tidal dispersion, and advection velocity; used for classification of tidal rivers and estuarine systems.
Existing use
Use actually attained in the waterbody on or after November 28, 1975, whether or not it is included in the water quality standards (40 CFR 131.3).
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Fate of pollutants
Physical, chemical, and biological transformation in the nature and changes of the amount of a pollutant in an environmental system. Transformation processes are pollutant-specific. Because they have comparable kinetics, different formulations for each pollutant are not required.
A confined area for the controlled feeding of animals. Tends to concentrate large amounts of animal waste that cannot be absorbed by the soil and, hence, may be carried to nearby streams or lakes by rainfall runoff.
Fine particulate material such as silt and clay particles typically of less than .85 mm diameter
The process by which suspended colloidal or very fine particles are assembled into larger masses or floccules that eventually settle out of suspension.
Fluvial geomorphology
The effect of rainfall and runoff on the form and pattern of riverbeds and river channels.
Movement and transport of mass of any water quality constituent over a given period of time. Units of mass flux are mass per unit time.
Forcing functions
External empirical formulation used to provide input describing a number of processes. Typical forcing functions include parameters such as temperature, point and tributary sources, solar radiation, and waste loads and flow.
Young, newly hatched fish.
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Referring to chemical reactions involving earth materials such as soil, rocks, and water.
The study of the evolution and configuration of landforms.
The rate of change of the value of one quantity with respect to another; for example, the rate of decrease of temperature with depth in a lake.
Ground water
The supply of fresh water found beneath the earth's surface, usually in aquifers, which supply wells and springs. Because ground water is a major source of drinking water, there is growing concern over contamination from leaching agricultural or industrial pollutants and leaking underground storage tanks.
Gully erosion
The erosion process whereby water accumulates in narrow channels and, over short periods, removes the soil form this narrow area to considerable depths, ranging from 1-2 feet to as much as 75-100 feet.
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The place where a plant or animal lives.
Habitat corridor
An area of land, such as a linear drainage ditch, a hedgerow, or railway embankment, that connects islands of wildlife habitat. Habitat corridors may also be referred to as stream corridors, wildlife corridors, or riparian zones.
Habitat structure
Defined by the pool-riffle ratio, pool frequency, depth and substrate, habitat complexity, instream cover, riffle substrate quality, riparian vegetative cover, riffle embeddedness.
Hillslope targets
Quantitative measure that links the upslope sources of sediment and instream impacts of sediment discharge.
Hydrodynamic model
Mathematical formulation used in describing fluid flow circulation, transport, and deposition processes in receiving water.
A graph showing variation of in stage (depth) or discharge of water in a stream over a period of time.
Hydrologic cycle
The circuit of water movement from the atmosphere to the earth and its return to the atmosphere through various stages or processes, such as precipitation, interception, runoff, infiltration, storage, evaporation, and transpiration.
Hydrologic reserves
Undeveloped areas responsible for maintaining the predevelopment hydrologic response of a watershed. The three most common land uses are crops, forest, and pasture.
The study of the distribution, properties, and effects of water on the earth's surface, in the soil and underlying rocks, and in the atmosphere.
Graph of rainfall rate during a storm event.
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Index of biotic integrity (IBI)
The IBI uses measurements of the distribution and abundance or absence of several fish species types in each waterbody for comparison. A portion of a waterbody is compared to a similar, unimpacted waterbody in the same ecoregion.
A measurable quantity that can be used to evaluate the relationship between pollutant sources and their impact on water quality.
Infiltration capacity
The capacity of a soil to allow water to infiltrate into or through it during a storm.
In situ
In place; in situ measurements consist of measurements of components of processes in a full-scale system or a field, rather than in a laboratory.
Applying water or wastewater to land areas to supply the water and nutrient needs of plants.
Irrigation return flow
Surface and subsurface water that leaves a field after the application of irrigation water.
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Jackson turbidity units (JTU)
An alternative way (to NTU) to measure turbidity in water based on the length of a light path through a suspension that causes the image of a standard candle flame to disappear.
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Karst geology
Solution cavities and closely-spaced sinkholes formed as a result of dissolution of carbonate bedrock.
Kinetic processes
Description of the rates and modes of changes in the transformation or degradation of a substance in an ecosystem.
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Land trusts
Legal entities which can operate locally, regionally, or nationally designed to own titles to or conservation easements on specific properties. Land trusts provide a focused venue for characterizing, prioritizing, and purchasing land or easements, and have an excellent track record of achieving the benefits sought.
Water that collects contaminants as it trickles through wastes, pesticides, or fertilizers. Leaching can occur in farming areas, feedlots, and landfills, and can result in hazardous substances entering surface water, groundwater, or soil.
Least disturbed condition
The best available existing conditions with regard to physical, chemical, and biological characteristics or attributes of a water body within a class or region. These waters have the least amount of human disturbance in comparison to others within the water body class, region or basin. Least disturbed conditions can be readily found, but may depart significantly from natural, undisturbed conditions or minimally disturbed conditions. Least disturbed condition may change significantly over time as human disturbances change.
Light saturation
The optimal light level for algae and macrophyte growth and photosynthesis.
Loading, Load, Loading rate
The total amount of material (pollutants) entering the system from one or multiple sources; measured as a rate in weight per unit time.
Load allocation (LA)
The portion of a receiving water's loading capacity that is attributed either to one of its existing or future nonpoint sources of pollution or to natural background sources. Load allocations are best estimates of the loading, which can range from reasonably accurate estimates to gross allotments, depending on the availability of data and appropriate techniques for predicting the loading. Wherever possible, natural and nonpoint source loads should be distinguished. (40 CFR 130.2(g))
Loading capacity (LC)
The greatest amount of loading that a water can receive without violating water quality standards.
Longitudinal dispersion
The spreading of chemical or biological constituents, including pollutants, downstream from a point source at varying velocities due to the differential in-stream flow characteristics.
Low-flow (7Q10)
Low-flow (7Q10) is the 7-day average low flow occurring once in 10 years; this probability-based statistic is used in determining stream design flow conditions and for evaluating the water quality impact of effluent discharge limits.
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Margin of safety (MOS)
A required component of the TMDL that accounts for the uncertainty about the relationship between the pollutant loads and the quality of the receiving waterbody (CWA section 303(d)(1)(C)). The MOS is normally incorporated into the conservative assumptions used to develop TMDLs (generally within the calculations or models) and approved by EPA either individually or in state/EPA agreements. If the MOS needs to be larger than that which is allowed through the conservative assumptions, additional MOS can be added as a separate component of the TMDL (in this case, quantitatively, a TMDL = LC = WLA + LA + MOS).
Mass balance
An equation that accounts for the flux of mass going into a defined area and the flux of mass leaving the defined area. The flux in must equal the flux out.
Mass loading
The quantity of a pollutant transported to a waterbody.
Mass wasting
Downslope transport of soil and rocks due to gravitational stress.
Mathematical model
A system of mathematical expressions that describe the spatial and temporal distribution of water quality constituents resulting from fluid transport and the one, or more, individual processes and interactions within some prototype aquatic ecosystem. A mathematical water quality model is used as the basis for waste load allocation evaluations.
Maximum depth
The greatest depth of a waterbody.
Mean depth
Volume of a waterbody divided by its surface area.
The transformation of organic matter into a mineral or an inorganic compound.
Minimally disturbed
The physical, chemical, and biological conditions of a water body with very limited, or minimal, human disturbance in comparison to others within the water body class or region. Minimally disturbed conditions can change over time in response to natural processes.
Actions taken to avoid, reduce, or compensate for the effects of environmental damage. Among the broad spectrum of possible actions are those that restore, enhance, create, or replace damaged ecosystems.
Periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and/or pollutant levels in various media or in humans, plants, and animals.
Monte Carlo simulation
A stochastic modeling technique that involves the random selection of sets of input data for use in repetitive model runs. Probability distributions of receiving water quality concentrations are generated as the output of a Monte Carlo simulation.
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Narrative criteria
Nonquantitative guidelines that describe the desired water quality goals.
National Pollutant Discharge Elimination System (NPDES)
The national program for issuing, modifying, revoking and reissuing, terminating, monitoring, and enforcing permits, and imposing and enforcing pretreatment requirements, under Sections 307, 402, 318, and 405 of the Clean Water Act.
Natural waters
Flowing water within a physical system that has developed without human intervention, in which natural processes continue to take place.
Nephelometric turbidity units (NTU)
The units of measurement for turbidity in water as determined by the degree light is scattered at right angles when compared to a standard reference solution.
Nonpoint source
Pollution that is not released through pipes but rather originates from multiple sources over a relatively large area. Nonpoint sources can be divided into source activities related to either land or water use including failing septic tanks, improper animal-keeping practices, forest practices, and urban and rural runoff.
Numeric target
A measurable value determined for the pollutant of concern which, if achieved, is expected to result in the attainment of water quality standards in the listed waterbody.
Numerical model
Model that approximates a solution of governing partial differential equations which describe a natural process. The approximation uses a numerical discretization of the space and time components of the system or process.
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One-dimensional model (1-D)
A mathematical model defined along one spatial coordinate of a natural water system. Typically 1-D models are used to describe the longitudinal variation of water quality constituents along the downstream direction of a stream or river. In writing the model, it is assumed that the cross-channel (lateral) and vertical variability is relatively homogenous and can, therefore, be averaged over those spatial coordinates.
Organic matter
The organic fraction that includes plant and animal residue at various stages of decomposition, cells and tissues of soil organisms, and substance synthesized by the soil population. Commonly determined as the amount of organic material contained in a soil or water sample.
The point where water flows from a conduit, stream, or drain.
Oxygen demand
Measure of the dissolved oxygen used by a system (microorganisms) in the oxidation of organic matter. (See also Biochemical oxygen demand.)
Oxygen depletion
A deficit of dissolved oxygen in a water system due to oxidation of organic matter.
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Disease-causing agent, especially microorganisms such as bacteria, protozoa, and viruses.
Peak runoff
The highest value of the stage or discharge attained by a flood or storm event; also referred to as flood peak or peak discharge.
Microscopic underwater plants and animals that are firmly attached to solid surfaces such as rocks, logs, pilings, and other structures.
An authorization, license, or equivalent control document issued by EPA or an approved Federal, state, or local agency to implement the requirements of an environmental regulation; e.g., a permit to operate a wastewater treatment plant or to operate a facility that may generate harmful emissions.
Permit compliance system (PCS)
Computerized management information system which contains data on NPDES permit-holding facilities. PCS keeps extensive records on more than 65,000 active water-discharge permits on sites located throughout the nation. PCS tracks permit, compliance, and enforcement status of NPDES facilities.
Phased approach
Under the phased approach to TMDL development, LAs and WLAs are calculated using the best available data and information recognizing the need for additional monitoring data to accurately characterize sources and loadings. The phased approach is typically employed when nonpoint sources dominate. It provides for the implementation of load reduction strategies while collecting additional data.
Point source
Pollutant loads discharged at a specific location from pipes, outfalls, and conveyance channels from either municipal wastewater treatment plants or industrial waste treatment facilities. Point sources can also include pollutant loads contributed by tributaries to the main receiving water stream or river.
Dredged spoil, solid waste, incinerator residue, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials, heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal, and agricultural waste discharged into water. (CWA Section 502(6)).
Generally, the presence of matter or energy whose nature, location, or quantity produces undesired environmental effects. Under the Clean Water Act, for example, the term is defined as the man-made or maninduced alteration of the physical, biological, chemical, and radiological integrity of water.
Portion of a stream with reduced current velocity, often with deeper water than surrounding areas and with a smooth surface.
A subsequent examination and verification of model predictive performance following implementation of an environmental control program.
Primary productivity
A measure of the rate at which new organic matter is formed and accumulated through photosynthesis and chemosynthesis activity of producer organisms (chiefly, green plants). The rate of primary production is estimated by measuring the amount of oxygen released (oxygen method) or the amount of carbon assimilated by the plant (carbon method).
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The net flux of oxygen occurring from the atmosphere to a body of water with a free surface.
Receiving waters
Creeks, streams, rivers, lakes, estuaries, ground-water formations, or other bodies of water into which surface water and/or treated or untreated waste are discharged, either naturally or in man-made systems.
Nest made in gravel, consisting of a depression hydraulically dug by a fish for egg deposition (and then filled) and the associated gravel mounds.
Reference condition (biological integrity)
The condition that approximates natural, un-impacted conditions (biological, chemical, physical, etc.) for a water body. Reference condition (biological integrity) is best determined by collecting measurements at a number of sites in a similar waterbody class or region under undisturbed or minimally disturbed conditions (by human activity), if they exist. Since undisturbed or minimally disturbed conditions may be difficult or impossible to find, least disturbed conditions, combined with historical information, models or other methods may be used to approximate reference condition as long as the departure from natural or ideal is understood. Reference condition is used as a benchmark to determine how much other water bodies depart from this condition due to human disturbance.
   (1) Minimally disturbed condition. The physical, chemical, and biological conditions of a waterbody with very limited, or minimal, human disturbance in comparison to others within the waterbody class or region. Minimally disturbed conditions can change over time in response to natural processes.
   (2) Least disturbed condition. The best available existing conditions with regard to physical, chemical, and biological characteristics or attributes of a waterbody within a class or region. These waters have the least amount of human disturbance in comparison to others within the waterbody class, region or basin. Least disturbed conditions can be readily found, but may depart significantly from natural, undisturbed conditions or minimally disturbed conditions. Least disturbed condition may change significantly over time as human disturbances change.
Reference sites
Waterbodies that are representative of the characteristics of the region and subject to minimal human disturbance.
Regional reference condition
Description of the chemical, physical, or biological condition based on an aggregation of data from reference sites that are representative of a water body type in an ecoregion, subecoregion, watershed, or political unit.
Reserve capacity
Pollutant loading rate set aside in determining stream waste load allocation accounting for uncertainty and future growth.
Residence time
Length of time that a pollutant remains within a section of a stream or river. The residence time is determined by the streamflow and the volume of the river reach or the average stream velocity and the length of the river reach.
A rocky shoal or sand bar located just below the surface of the water.
Return of an ecosystem to a close approximation of its presumed condition prior to disturbance.
Rill erosion
An erosion process in which numerous small channels of only several centimeters in depth are formed; occurs mainly on recently cultivated soils.
Riparian areas
Areas bordering streams, lakes, rivers, and other watercourses. These areas have high water tables and support plants that require saturated soils during all or part of the year. Riparian areas include both wetland and upland zones.
Riparian vegetation
Hydrophytic vegetation growing in the immediate vicinity of a lake or river closely enough so that its annual evapotranspiration constitutes a factor in the lake or river regime.
Riparian zone
The border or banks of a stream. Although this term is sometimes used interchangeably with floodplain, the riparian zone is generally regarded as relatively narrow compared to a floodplain. The duration of flooding is generally much shorter, and the timing less predictable, in a riparian zone than in a river floodplain.
Roughness coefficient
A factor in velocity and discharge formulas representing the effects of channel roughness on energy losses in flowing water. Manning's "n" is a commonly used roughness coefficient.
That part of precipitation, snow melt, or irrigation water that runs off the land into streams or other surface water. It can carry pollutants from the air and land into receiving waters.
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Sanitary sewer overflows
Properly designed, operated, and maintained sanitary sewer systems are meant to collect and transport all of the sewage that flows into them to a publicly owned treatment works (POTW). However, occasional unintentional discharges of raw sewage from municipal sanitary sewers occur in almost every system. These types of discharges are called sanitary sewer overflows.
Scoping modeling
A method of approximation that involves simple, steady-state analytical solutions for a rough analysis of a problem.
To abrade and wear away. Used to describe the weathering away of a terrace or diversion channel or streambed. The clearing and digging action of flowing water, especially the downward erosion by stream water in sweeping away mud and silt on the outside of a meander or during flood events.
Particulate organic and inorganic matter that accumulates in a loose, unconsolidated form on the bottom of natural waters.
Sediment delivery
Contribution of transported sediment to a particular location or part of a landscape.
Sediment oxygen demand (SOD)
The solids discharged to a receiving water are partly organics, and upon settling to the bottom, they decompose anaerobically as well as aerobically, depending on conditions. The oxygen consumed in aerobic decomposition represents another dissolved oxygen sink for the waterbody.
Sediment production
Delivery of colluvium or bedrock from hillslope to stream channel. The production rate is evaluated as the sum of the rates of colluvial bank erosion and sediment transport across channel banks.
Sediment yield
Amount of sediment passing a particular point (e.g., discharge point of the basin) in a watershed per unit of time.
Process of deposition of waterborne or windborne sediment or other material; also refers to the infilling of bottom substrate in a waterbody by sediment (siltation).
Settleable solids
Those solids that will settle to the bottom of a cone-shaped container, an Imhoff cone, in a 60-minute period.
A channel or conduit that carries wastewater and stormwater runoff from the source to a treatment plant or receiving stream. "Sanitary" sewers carry household, industrial, and commercial waste. "Storm" sewers carry runoff from rain or snow. "Combined" sewers handle both.
Sheet erosion
Also Sheetwash. Erosion of the ground surface by unconcentrated (i.e. not in rills) overland flow.
Also sheet erosion. Erosion of the ground surface by unconcentrated (i.e. not in rills) overland flow.
Noncohesive soil whole individual particles are not visible to the unaided human eye (0.002 to 0.05 mm). Silt will crumble when rolled into a ball.
The process by which a river, lake, or other water body becomes clogged with sediment.
The use of mathematical models to approximate the observed behavior of a natural water system in response to a specific known set of input and forcing conditions. Models that have been validated, or verified, are then used to predict the response of a natural water system to changes in the input or forcing conditions.
The degree to which a river or stream bends.
The degree of inclination to the horizontal. Usually expressed as a ratio, such as 1:25 or 1 on 25, indicating one unit vertical rise in 25 units of horizontal distance, or in a decimal fraction (0.04); degrees (2 degrees 18 minutes), or percent (4 percent).
Source water
Untreated water from streams, rivers, lakes, or underground aquifers which is used to supply private wells and public drinking water.
Spatial segmentation
A numerical discretization of the spatial component of a system into one or more dimensions; forms the basis for application of numerical simulation models.
Patterns of urban growth which includes large acreage of low-density residential development, rigid separation between residential and commercial uses, residential and commercial development in rural areas away from urban centers, minimal support for non-motorized transportation methods, and a lack of integrated transportation and land use planning.
Stabilization pond
Large earthen basin used for the treatment of wastewater by natural processes involving the use of both algae and bacteria.
Steady-state model
Mathematical model of fate and transport that uses constant values of input variables to predict constant values of receiving water quality concentrations.
Stoichiometric ratio
Mass-balance-based ratio for nutrients, organic carbon and algae (e.g., nitrogen-to-carbon ratio).
U.S. Environmental Protection Agency (EPA) national water quality database for STORage and RETrieval (STORET). Mainframe water quality database that includes physical, chemical, and biological data measured in waterbodies throughout the United States.
Storm runoff
Storm water runoff, snowmelt runoff, and surface runoff and drainage; rainfall that does not evaporate or infiltrate the ground because of impervious land surfaces or a soil infiltration rate lower than rainfall intensity, but instead flows onto adjacent land or waterbodies or is routed into a drain or sewer system.
Stratification (of waterbody)
Formation of water layers each with specific physical, chemical, and biological characteristics. As the density of water decreases due to surface heating, a stable situation develops with lighter water overlaying heavier and denser water.
Discharge that occurs in a natural channel. Although the term "discharge" can be applied to the flow of a canal, the word "streamflow" uniquely describes the discharge in a surface stream course. The term streamflow is more general than "runoff" as streamflow may be applied to discharge whether or not it is affected by diversion or regulation.
Stream hydrology
The flow regime of a stream. Several variables of hydrology can be altered by construction, development, or land conversion including summer dry weather flow, wetted perimeter, cross-sectional area of the stream, and peak storm flow. Increased runoff can increase flood peaks and the magnitude and frequency of bankfull storms, and decrease baseflow between storms.
Stream restoration
Various techniques used to replicate the hydrological, morphological, and ecological features that have been lost in a stream due to urbanization, farming, or other disturbance.
Any physical, chemical, or biological entity that can induce an adverse response.
Refers to bottom sediment material in a natural water system.
Surface area
The area of the surface of a waterbody; best measured by planimetry or the use of a geographic information system.
Surface runoff
Precipitation, snowmelt, or irrigation water in excess of what can infiltrate the soil surface and be stored in small surface depressions; a major transporter of nonpoint source pollutants.
Surface water
All water naturally open to the atmosphere (rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc.) and all springs, wells, or other collectors directly influenced by surface water.
Suspended and bedded sediments
Particulate organic and inorganic matter that suspend in or are carried by the water, and/or accumulate in a loose, unconsolidated form on the bottom of natural water bodies.
Suspended solids or load
Organic and inorganic particles (sediment) suspended in and carried by a fluid (water). The suspension is governed by the upward components of turbulence, currents, or colloidal suspension. Suspended sediment usually consists of particles <0.1 mm, although size may vary according to current hydrological conditions. Particles between 0.1 mm and 1 mm may move as suspended or be deposited (bedload).
Suspended solids concentration (SSC)
The amount of organic and inorganic particles suspended in water. SSC is determined by measuring the dry weight of all the sediment from a known volume of a water-sediment mixture.
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Deepest part of a stream channel.
Three-dimensional model (3-D)
Mathematical model defined along three spatial coordinates where the water quality constituents are considered to vary over all three spatial coordinates of length, width, and depth.
The physical features of a geographic surface area including relative elevations and the positions of natural and man-made features.
Total maximum daily load (TMDL)
The sum of the individual wasteload allocations (WLAs) for point sources, load allocations (LAs) for nonpoint sources and natural background, plus a margin of safety (MOS). TMDLs can be expressed in terms of mass per time, toxicity, or other appropriate measures that relate to a state's water quality standard.
Total suspended solids (TSS)
The entire amount of organic and inorganic particles dispersed in water. TSS is measured by several methods, most of which entail measuring the dry weight of sediment from a known volume of a subsample of the original.
Transit time
In nutrient cycles, the average time that a substance remains in a particular form; ratio of biomass to productivity.
Transport of pollutants (in water)
Transport of pollutants in water involves two main processes: (1) advection, resulting from the flow of water, and (2) diffusion, or transport due to turbulence in the water.
A lower order stream compared to a receiving waterbody. "Tributary to" indicates the largest stream into which the reported stream or tributary flows.
   (1) The scattering of light by fine, suspended particles which causes water to have a cloudy appearance. Turbidity is an optical property of water. More specifically, turbidity is the intensity of light scattered at one or more angles to an incident beam of light as measured by a turbidity meter or nephelometer.
   (2) A principal characteristic of water clarity,expressed as the optical property of water that causes light to be absorbed by particles and molecules rather than be transmitted in straight lines through a water sample. It is caused by suspended matter or impurities that interfere with the clarity of water. These impurities may include clay, silt, finely divided inorganic and organic matter, soluble colored organic compounds, and plankton and other microscopic organisms.
Turbulent flow
A flow characterized by agitated and irregular, random-velocity fluctuations.
A type of flow in which any particle may move in any direction with respect to any other particle and not in a smooth or fixed path. Turbulent water is agitated by cross current and eddies. Turbulent velocity is that velocity above which turbulent flow will always exist and below which the flow may be either turbulent or laminar.
Two-dimensional model (2-D)
A mathematical model defined along two spatial coordinates where the water quality constituents are considered averaged over the third remaining spatial coordinate. Examples of 2-D models include descriptions of the variability of water quality properties along: (a) the length and width of a river that incorporates vertical averaging of depth, or (b) length and depth of a river that incorporates lateral averaging across the width of the waterbody.
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Uncertainty factors
Factors used in the adjustment of toxicity data to account for unknown variations. Where toxicity is measured on only one test species, other species may exhibit more sensitivity to that effluent. An uncertainty factor would adjust measured toxicity upward and downward to cover the sensitivity range of other, potentially more or less sensitive species.
Indicates a vertically uniform or well-mixed condition in a waterbody. See also stratified.
Use attainability analysis (UAA)
A structured scientific assessment of the factors affecting the attainment of the use which may include physical, chemical, and economic factors as described in section 131.10(g). (40 CFR 131.3)
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Validation (of a model)
Process of determining how well the mathematical model's computer representation describes the actual behavior of the physical process under investigation.
Verification (of a model)
Testing the accuracy and predictive capabilities of the calibrated model on a data set independent of the data set used for calibration.
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Sediments smaller than 63 microns which are not from the bed but could be from bank erosion or upland sources.
Wasteload allocation (WLA)
The portion of a receiving water's loading capacity that is allocated to one of its existing or future point sources of pollution. WLAs constitute a type of water quality-based effluent limitation (40 CFR 130.2(h)).
Usually refers to effluent from a sewage treatment plant. See also domestic wastewater.
Wastewater treatment
Chemical, biological, and mechanical procedures applied to an industrial or municipal discharge or to any other sources of contaminated water in order to remove, reduce, or neutralize contaminants.
Water quality
The biological, chemical, and physical conditions of a waterbody. It is a measure of a waterbody's ability to support beneficial uses.
Water quality-based effluent limitations
Effluent limitations applied to dischargers when mere technology-based limitations would cause violations of water quality standards. Usually WQBELs are applied to discharges into small streams.
Water quality-based permit
A permit with an effluent limit more stringent than one based on technology performance. Such limits may be necessary to protect the designated use of receiving waters (e.g., recreation, irrigation, industry or water supply).
Water quality criteria
Levels of water quality expected to render a body of water suitable for its designated use, composed of numeric and narrative criteria. Numeric criteria are scientifically derived ambient concentrations developed by EPA or states for various pollutants of concern to protect human health and aquatic life. Narrative criteria are statements that describe the desired water quality goal. 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-limited segments.
Those water segments which do not or are not expected to meet applicable water quality standards even after the application of technology-based effluent limitations required by sections 301(b) and 306 of the Clean Water Act (40 CFR 130.29(j)). Technology-based controls include, but are not limited to, best practicable control technology currently available (BPT) and secondary treatment.
Water quality standard.
Law or regulation that consists of the beneficial designated use or uses of a waterbody, the numeric and narrative water quality criteria that are necessary to protect the use or uses of that particular waterbody, and an anti-degradation statement.
Water quality-limited segments
Those water segments which do not or are not expected to meet applicable water quality standards even after the application of technology-based effluent limitations required by sections 301(b) and 306 of the Clean Water Act (40 CFR 130.29(j)). Technology-based controls include, but are not limited to, best practicable control technology currently available (BPT) and secondary treatment.
Water quality management plans
Prescribe the regulatory, construction, and management activities necessary to meet the water body goals.
Water quality standards
Provisions in State or Tribal law or regulations that define the water quality goals of a water body, or segment thereof, by designating the use or uses to be made of the water; setting criteria necessary to protect the uses; and protecting existing water quality through anti-degradation policies and implementation procedures.
Watershed-based trading
Watershed-based trading is an efficient, market-driven approach that encourages innovation in meeting water quality goals, but remains committed to enforcement and compliance responsibilities under the Clean Water Act. It involves trading arrangements among point source dischargers, nonpoint sources, and indirect dischargers in which the "buyers" purchase pollutant reductions at a lower cost than what they would spend to achieve the reductions themselves. Sellers provide pollutant reductions and may receive compensation. The total pollution reduction, however, must be the same or greater than what would be achieved if no trade occurred.
Watershed protection approach (WPA)
The USEPA's comprehensive approach to managing water resource areas, such as river basins, watersheds, and aquifers. WPA has four major features—targeting priority problems, stakeholder involvement, integrated solutions, and measuring success.
Watershed-scale approach
A consideration of the entire watershed, including the land mass that drains into the aquatic ecosystem.
A drainage area or basin in which all land and water areas drain or flow toward a central collector such as a stream, river, or lake at a lower elevation.
An area that is saturated by surface water or ground water with vegetation adapted for life under those soil conditions, as in swamps, bogs, fens, marshes, and estuaries.
APSRS    Aerial Photography Summary Record System
Bank Erosion for Nonpoint Source Consequences of Sediment
Bank Erosion Hazard Index
Bank Height
Bank Height Ratio
Belt Width
Channel Enlargement Potential
Cubic Feet per Second
Channel Slope
1) Depth
2) Diameter of dominant bed particles (as in D50, D84 etc)
Ephemeroptera, Plecoptera, & Trichoptera (aquatic invertebrates)
Federal Interagency Stream Restoration Working Group
Flood Prone Area
Forest Service Water Erosion Prediction Procedure
Gravitational Acceleration
Geographic Information System
Global Positioning System
Generalized Sediment Transport for Alluvial Rivers
Generalized Sediment Transport for Alluvial Rivers & Watersheds
Idaho Dept of Environmental Quality
Meander Length
Meander Arc Length
Meander Width Ratio
Near Bank Stress
Natural Resources Conservation Service
Prediction Level Assessment\
Pacific Southwest Inter-agency Committee
Discharge (flow)
Hydraulic Radius
Radius of Curvature
Reconnaissance Level Assessment
Road Risk Index
Rapid Resource Inventory for Sediment & Stability Consequences
Revised Universal Soil Loss Equation
Suspended & Bedded Sediments
Submerged Aquatic Vegetation
Bedload Sediment
Soil Conservation Service
Severity of Ill Effect
Stream Length
Sediment Rating Curve
Total Maximum Daily Load
Total Suspended Solids
U. S. Dept of Agriculture
U. S. Environmental Protection Agency
U. S. Geological Survey
Universal Soil Loss Equation
Valley Length
Valley Slope
Width/Depth Ratio
Watershed Assessment of River Stability & Sediment Supply
Water Erosion Prediction Project
Wetted Perimeter
Water Quality Standard
Water Resource Evaluation of Nonpoint Silvicultural Sources

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