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Glossary of stream, lake and watershed terms

Glossary of stream, lake and watershed terms

Glossary of stream, lake and watershed terms

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303(d) waterbody

A list of lakes, rivers, and streams that have been designated as impaired or threatened by a pollutant(s) for which one or more TMDL(s) are needed. Impaired means that the water is not meeting state water quality standards.


Mostly aquatic, non-vascular plants that float in the water or attach to larger plants, rocks, and other substrates. Also called phytoplankton, these individuals are usually visible only with a microscope. They are a normal and necessary component of aquatic life, but excessive numbers can make the water appear cloudy and colored.

Alluvial fan

A fan-shaped accumulation of alluvium deposited at the mouth of a ravine or at the juncture of a tributary stream with the main stream.

The acid-neutralizing capacity of water. It is primarily a function of the carbonate, bicarbonate, and hydroxide content in water. The lower the alkalinity, the less capacity the water has to absorb acids without becoming more acidic.

Ammonia (NH3)

A nitrogen-containing substance which may indicate recently decomposed plant or animal material.

Anadromous fish

Fish that ascend rivers from the sea for breeding.


A geologic stratum containing groundwater that can be withdrawn and used for human purposes.



Any area draining to a point of interest. Basins of interest to King County staff are those that drain either to the Cedar, Green, Snoqualmie, Skykomish, or White rivers, or the drainage areas which drain directly to Puget Sound.

Basin plan

A plan and all implementing regulations and procedures including but not limited to capital projects, public education activities, land use management regulations adopted by ordinance for managing surface and storm water management facilities, and features within individual subbasins.

The communities of aquatic life which dwell in or on the bottom sediments of a water body.

Biofiltration swale or Bioswale sketch (PDF, 400-800kb)

A long, gently sloped, vegetated ditch designed to filter pollutants from stormwater. Grass is the most common vegetation, but wetland vegetation can be used if the soil is saturated.


Best Management Practice. Also see Structural and Nonstructural BMPs.


A designated area adjacent to and a part of a steep slope or landslide hazard area which protects slope stability, attenuation of surface water flows, and landslide hazards reasonably necessary to minimize risk; or a designated area adjacent to or a part of a stream or wetland that is an integral part of the stream or wetland ecosystem.



A long, narrow excavation or surface feature that conveys surface water and is open to the air.

Channel, constructed

A channel or ditch constructed to convey surface water; also includes reconstructed natural channels.

Channel, natural

a channel which has occurred naturally due to the flow of surface waters; or a channel that, although originally constructed by human activity, has taken on the appearance of a natural channel including a stable route and biological community.


Pigments (mostly green) in plants, including algae, that play an important part in the chemical reactions of photosynthesis. A measurement of chlorophyll-a (one type of chlorophyll) is commonly used as a measure of the algae content of water.

Closed depression

An area which is low-lying and either has no surface water outlet, or has such a limited outlet that during storm events the area acts as a retention basin, with more than 5000 square feet of water surface area at overflow elevation.

Combined Sewer Overflow (CSO)

Discharges of combined sewage and stormwater into water bodies during very wet or storm weather. These discharges occur to relieve the sewer system as it becomes overloaded with normal sewer flow and increased storm run-off. The term is also used to denote a pipe that discharges those overflows.

Conductivity (Cond)

A measure of water's capacity to convey an electric current. It is related to the total amount of dissolved charged substances in the water. Therefore, it can be used as a general indicator of the quality of the water and can also suggest presence of unidentified material in the water. It is often used as a surrogate for salinity measurements.

Constructed conveyance system facilities

Gutters, ditches, pipes, channels, and most flow control and water quality treatment facilities.

Conveyance System

Drainage facilities and features that collect, contain, and provide for the flow of surface and storm water from the highest points on the land down to a receiving water. Conveyance systems are made up of natural elements and of constructed facilities.

Critical Drainage Area

An area with such severe flooding, drainage, and/or erosion/sedimentation conditions which have resulted or will result from the cumulative impacts of development and urbanization, that the area has been formally adopted as a Critical Drainage Area by rule under the procedures specified in KCC 2.98.



Release of surface and storm water runoff from the site at a slower rate than it is collected by the drainage facility system, the difference being held in temporary storage.

Detention facility

A facility that collects water from developed areas and releases it at a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored in a pond or a vault and is typically released over a few hours or a few days.

Direct discharge

Undetained discharge from a proposed project to a major receiving water.


Runoff, excluding offsite flows, leaving the proposed development through overland flow, built conveyance systems, or infiltration facilities.

Dissolved oxygen (DO)

Oxygen that is dissolved in the water. Certain amounts are necessary for life processes of aquatic animals. The oxygen is supplied by the photosynthesis of plants, including algae, and by aeration. Oxygen is consumed by animals and plants at night, and bacterial decomposition of dead organic matter (plant matter and animal waste).

Dispersed discharge

Release of surface and storm water runoff from a drainage facility system such that the flow spreads over a wide area and is located so as not to allow flow to concentrate anywhere upstream of a drainage channel with erodible underlying granular soils or the potential to flood downstream properties.


A constructed channel with its top width less than 10 feet at design flow.


A change in the natural discharge location or runoff flows onto or away from an adjacent downstream property.


The collection, conveyance, containment, and/or discharge of surface and storm water runoff.

Drainage area or Drainage basin

An area draining to a point of interest.

Drainage facility

A constructed or engineered feature that collects, conveys, stores or treats surface and storm water runoff. Drainage facilities shall include but not be limited to all constructed or engineered streams, pipelines, channels, ditches, gutters, lakes, wetlands, closed depressions, flow control or water quality treatment facilities, erosion and sedimentation control facilities, and other drainage structures and appurtenances that provide for drainage.



Liquids discharged from sewage treatment plants, septic systems, or industrial sources to surface waters.


A structure of earth, gravel, or similar material raised to form a pond bank or foundation for a road.


The warmer, well-lit surface waters of a lake that are thermally separated from the colder (hence denser), water at the bottom of the lake when a lake is stratified.


The acceleration of the loading of nutrients to a lake by natural or human-induced causes. The increased rate of delivery of nutrients results in increased production of algae and consequently, poor water transparency. Human-induced (cultural) eutrophication may be caused by input of treated sewage to a lake, deforestation of a watershed, or the urbanization of a watershed.


The detachment and transport of soil or rock fragments by water, wind, ice, etc.


Erosion and Sediment Control


A condition of a water body in which excess nutrients, particularly phosphorous, stimulates the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen. Thus, less dissolved oxygen is available to other aquatic life.


Fecal Coliform Bacteria

Bacteria from the intestines of warm-blooded animals. Most of the bacteria are not in themselves harmful, so they are measured or counted as an indicator of the possible presence of harmful bacteria.


A type of wetland that forms peat deposits. Peat comes from the plants (especially Sphagnum moss) that grow, die, and accumulate in the fen. Bogs and fens are similar in that they both produce peat; however, bogs are essentially closed to water input other than rainwater, while fens generally have a surface water flow. Because of this hydrologic difference, fens are less acidic than bogs and may support different plants and animals.

Flow control facility

A drainage facility designed to mitigate the impacts of increased surface and storm water runoff generated by site development pursuant to the drainage requirements in King County Code Chapter 9.04. Flow control facilities are designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold runoff a short period of time and then release it to the conveyance system.



Water stored beneath the surface of the earth. The water in the ground is supplied by the seepage of rainwater, snowmelt, and other surface water into the soil. Some groundwater may be found far beneath the earth surface, while other groundwater may be only a few inches from the surface. Groundwater discharges into lowland streams to maintain their baseflow.



The specific area or environment in which a particular type of plant or animal lives and grows.


A cemented or compacted and often clay-like layer of soil that is impenetrable by roots.

Harmful pollutant

A substance that has adverse effects to an organism including death, chronic poisoning, impaired reproduction, cancer, or other effects.


The science dealing with the properties, distribution and circulation of water. The term usually refers to the flow of water on or below the land surface before reaching a stream or man-made structure.


The dark, cold, bottom waters of a lake that are thermally separated from the warmer (hence less dense) surface waters when a lake is stratified.


Impervious surface

A hard surface area which either prevents or retards the entry of water into the soil mantle as under natural conditions prior to development; and/or a hard surface area which causes water to run off the surface in greater quantities or at an increased rate of flow from the flow present under natural conditions prior to development.
Common impervious surfaces include, but are not limited to, roof tops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and oiled, macadam, or other surfaces which similarly impede the natural infiltration of surface and storm water runoff. Open, uncovered flow control or water quality treatment facilities shall not be considered impervious surfaces for determinations of thresholds. For the purpose of modeling though, onsite flow control and water quality ponds are modeled as impervious surface per Chapter 3 of the King County Surface Water Design Manual.

Infiltration facility

A drainage facility designed to use the hydrologic process of water soaking into the ground (commonly referred to as percolation) to dispose of surface and storm water runoff.


Animals without internal skeletons. Some require magnification to be seen well, while others such as worms, insects, and crayfish are relatively large. Invertebrates living in stream and lake sediments are collected as samples to be identified and counted. In general, more varied invertebrate communities indicate healthier water bodies.


Joint problems

Problems that involve more than one jurisdiction or which impact the sustainability and functionality of the drainage basin.



An area permanently inundated by water in excess of two meters (7 feet) deep and greater than twenty acres in size as measured at the ordinary high water mark.

Limiting nutrient

The nutrient that is in lowest supply relative to the demand. The limiting nutrient will be exhausted first by algae which require many nutrients and light to grow. Inputs of the limiting nutrient will result in increased algal production, but as soon as the limiting nutrient is exhausted, growth stops. Phytoplankton growth in lake waters of temperate lowland areas is generally phosphorus limited.


Scientific study of inland waters.

Littoral zone

Portion of a water body extending from the shoreline lakeward to the greatest depth occupied by rooted plants.

Loading rate
Addition of a substance to a water body; or the rate at which the addition occurs. For example, streams load nutrients to lakes at various rates as in "500 kilograms per year (500 kg/yr)" or "227 pounds per year (227 lb/yr)."

Local problems

Problems that impact only one jurisdiction and only a small isolated portion of the basin. Local problems also have relatively minor environmental impacts when viewed on a basin-wide scale, and are not expected to impact the overall sustainability and functionality of the drainage basin.


rooted and floating aquatic plants, larger (macro-) than the phytoplankton.


Maximum extent practicable

A condition of lakes that is characterized by moderate concentrations of nutrients, algae, and water transparency. A mesotrophic lake is not as rich in nutrients as a eutrophic lake, but richer in nutrients than an oligotrophic lake.

A lake which has one mixing and one stratification event per year. If a lake does not freeze over in the winter, the winter winds will mix the waters of the lake. In summer, the lake resists mixing and becomes stratified because the surface waters are warm (light) and the bottom waters are cold (dense). Deep lakes in the Puget lowlands are monomictic lakes.


Natural conveyance system elements

Swales and small drainage courses, streams, rivers, lakes, and wetlands.

Nitrate, nitrite (NO3, NO2)
Two types of nitrogen compounds. These nutrients are forms of nitrogen that algae may use for growth.

One of the elements essential as a nutrient for growth of organisms.

Non-point source pollution

Pollution that originates from diffuse areas and unidentifiable sources, such as agriculture, the atmosphere, or ground water.

Nonstructural BMP

A preventative action to protect receiving water quality that does not require construction. Nonstructural BMPs rely predominantly on behavioral changes in order to be effective. Major categories of non-structural BMPs include education, recycling, maintenance practices and source controls.


National Pollutant Discharge Elimination System. The part of the Clean Water Act which requires point source discharges to obtain permits. These permits, referred to as NPDES permits, are administered by the Washington State Department of Ecology.

Elements or compounds essential for growth of organisms.



A condition of lakes characterized by low concentrations of nutrients and algae and resulting good water transparency. An oligotrophic lake has less nutrients than a mesotrophic or eutrophic lake.


A point where collected and concentrated surface and storm water runoff is discharged from a pipe system or culvert.



Microorganisms that can cause disease in other organisms or humans, animals, and plants. Pathogens include bacteria, viruses, fungi, or parasites found in sewage, in runoff from farms or city streets, and in water used for swimming. Pathogens can be present in municipal, industrial, and nonpoint source discharges.

Pelagic Zone

Deep, open water area of a lake away from the edge of the littoral zone towards the center of the lake.


An unconsolidated deposit of semicarbonized plant remains, generally found in a watershed environment such as a bog or a fen. It generally has a moisture content of at least 75%, but when it is dried out, it burns very easily.


An area of vegetated matter with extensive peat deposits.

Physiographic subdivisions

Physiographic (or geomorphic) subdivisions are based on terrain texture, rock type, and geologic structure and history.

Measure of the acidity of water on a scale of 0 to 14, with 7 representing neutral water. A pH less than 7 is considered acidic and above 7 is basic.

One of the elements essential as a nutrient for the growth of organisms. In western Washington lakes, it is usually the algae nutrient in shortest supply relative to the needs of the algae. Phosphorus occurs naturally in soils, as well as in organic material. Various measures of phosphorus in water samples are made, including total-phosphorus (TP) and the dissolved portion of the phosphorus (orthophosphorus).

Photic zone

The lighted region of a lake where photosynthesis occurs.


Floating, mostly microscopic algae (plants) that live in water.

Point discharge

The release of collected and/or concentrated surface and storm water runoff from a pipe, culvert, or channel.

Point-source Pollution
An input of pollutants into a water body from discrete sources, such as municipal or industrial outfalls.

Primary Treatment

The first stage of wastewater treatment involving removal of debris and solids by screening and settling.

Pump Station

A structure used to move wastewater uphill, against gravity.



A small stream channel, narrow and steep-sided in cross section.


A length of channel with uniform characteristics.

Receiving waters

Bodies of water or surface water systems receiving water from upstream man-made or natural systems.

Regional problems

See joint problems


A structure that controls the flow of wastewater from two or more input pipes to a single output. Regulators can be used to restrict or halt flow, thus causing wastewater to be stored in the conveyance system until it can be handled by the treatment plant.

Resource stream

A stream section mapped and rated by King County as being a regionally significant stream reach that harbors significant concentrations of fish for some period in their life cycle.


The process of collecting and holding surface and storm water runoff with no surface outflow.


Pertaining to the banks of rivers and streams, and sometimes also wetlands, lakes, or tidewater.


A facing layer or protective mound of stones placed to prevent erosion or sloughing of a structure or embankment due to the flow of surface and storm water runoff.


Water originating from rainfall and other precipitation that ultimately flows into drainage facilities, rivers, streams, springs, seeps, ponds, lakes, and wetlands as well as shallow groundwater.



A member of the fish family Salmonidae. In King County salmonid species include Chinook, Coho, chum, sockeye, and pink salmon; cutthroat, rainbow, and brown trout and steelhead; Dolly Varden, brook trout, char, kokanee, and whitefish.

Secchi depth

Measure of transparency of water obtained by lowering a 10 cm black and white disk into water until it is no longer visible.

Secondary Treatment

Following primary treatment, bacteria are used to consume organic wastes. Wastewater is then disinfected and discharged through an outfall.


A method for controlling combined sewer overflow whereby the combined sewer is separated into both a sanitary sewer and a storm drain, as is the practice in new development.


That portion of wastewater that is composed of human and industrial wastes from homes, businesses, and industries.


A legally established allowable limit for a substance or characteristic in the water, based on criteria. Enforcement actions by the appropriate agencies can be taken against parties who cause violations.

Stratification of lakes

A layering effect produced by the warming of the surface waters in many lakes during summer. Upper waters are progressively warmed by the sun and the deeper waters remain cold. Because of the difference in density (warmer water is lighter), the two layers remain separate from each other: upper waters "float" on deeper waters and wind induced mixing occurs only in the upper waters. Oxygen in the bottom waters may become depleted. In autumn as the upper waters cool, the whole lake mixes again and remains mixed throughout the winter, or until it freezes over.


Water that is generated by rainfall and is often routed into drain systems.

Sphagnum bog wetlands

Unique wetlands having a predominance of sphagnum moss creating a substrate upon which a distinctive community of plants is established. Some of these include ledum groenlandicum (Labrador tea), Kalmia occidentalis (bog laurel), Drosera rotundifolia (sundew), and Vaccinium oxycoccos (cranberry). Stunted evergreen trees are also sometimes present. In addition to a distinctive plant community, the water chemistry of sphagnum wetlands is also unique. It is characterized by acidic waters (pH 3 to 5.5), low nutrient content, low alkalinity, and a buffering system composed predominantly of organic acids. In the Puget Sound area, mature sphagnum bog wetlands are typically very old, often dating back thousands of years.


Stormwater is the water that runs off surfaces such as rooftops, paved streets, highways, and parking lots. It can also come from hard grassy surfaces like lawns, play fields, and from graveled roads and parking lots.

Stormwater Facility

Facilities that control the discharge of stormwater and that remove pollutants make up the bulk of the structural solutions applied to surface water problems in King County. Stormwater facilities included storage facilities (ponds, vaults, underground tanks, and infiltration systems); water quality facilities (wetponds, biofiltration swales, constructed wetlands, sand filters, and oil/water separators); and conveyance systems (ditches, pipes, and catchbasins).

These systems are most often built in conjunction with new development, but include regional facilities designed and constructed by the Department of Natural Resources.

Once constructed, stormwater facilities require on-going maintenance to ensure they continue to perform as intended. Maintenance of storage facilities typically includes the removal of accumulated sediment and debris, routine mowing, and minor repairs to mechanical appurtenances. Management of water quality facilities is more complex, requiring intensive vegetation management, inspection and maintenance of flow control features, and restoration or replacement of filter media. King County plays an active role in the management of three categories of stormwater facilities: residential, commercial, and regional.

Stormwater Facility, Regional

In King County, regional stormwater facilities are constructed and/or managed by DNR's WLR Division. They typically serve large areas with a variety of land uses, and are intended to address problems resulting from large storm events. Examples of regional facilities include pump stations, regional storage facilities, sedimentation ponds, and enclosed drainage systems. These facilities are inspected annually and maintained by DNR's WLR Division.

Stormwater Management

The application of site design principles and construction techniques to prevent sediments and other pollutants from entering surface or ground water; source controls; and treatment of runoff to reduce pollution.

Storm drain system

The system of gutters, pipes, streams, or ditches used to carry surface and storm water from surrounding lands to streams, lakes, or Puget Sound. Also see Conveyance System.

Structural BMP

Constructed facilities or measures to help protect receiving water quality and control stormwater quantity. Examples include storage, vegetation, infiltration, and filtration.

Surface Water Design Manual

The manual (and supporting documents as appropriate) describing surface and storm water design and analysis requirements, procedures, and guidance which has been formally adopted by rule under the procedures specified in KCC 2.98. The Surface Water Design Manual is available from the King County Department of Development and Environmental Services or the Department of Natural Resources.


A shallow drainage conveyance with relatively gentle side slopes, generally with flow depths less than one foot.



Typically a continuous length of pipe used to convey flows down a steep or sensitive slope with appropriate energy dissipation at the discharge end.


A TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. Water quality standards identify the uses for each waterbody, for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing), and the scientific criteria to support that use. The Clean Water Act, section 303, establishes the water quality standards and TMDL programs.

Total suspended solids (TSS)

Particles, both mineral (clay and sand) and organic (algae and small pieces of decomposed plant and animal material), that are suspended in water.


Causing death, disease, cancer, genetic mutations, or physical deformations in any organism or its offspring upon exposure, ingestion, inhalation, or assimilation. Poisonous, carcinogenic, or otherwise directly harmful to life.


A measure of the clarity of water in a lake, which is measured by lowering a standard black and white Secchi disk into the water and recording the depth at which it is no longer visible. Transparency of lakes is determined by the color of the water and the amount of material suspended in it. Generally in colorless waters of the Puget lowland, the transparency of the water in summer is determined by the amount of algae present in the water. Suspended silt particles may also have an effect, particularly in wet weather.

Trophic status
Rating of the condition of a lake on the scale of oligotrophic-mesotrophic-eutrophic (see definition of these terms).

Cloudiness of water caused by the suspension of minute particles, usually algae, silt, or clay.



An area of high or relatively high ground



Total flow within the sewage system. In combined systems, it includes sewage and stormwater.

Water Column

Water in a lake between the surface and sediments. Used in vertical measurements used to characterize lake water.


The areas that drain to surface water bodies, including lakes, rivers, estuaries, wetlands, streams, and the surrounding landscape.

Water of Statewide Significance

Legal term from the state Shoreline Management act, which recognizes particular bodies of water and sets criteria and standards for their protection.

Water quality treatment facility

A drainage facility designed to reduce pollutants once they are already contained in surface and storm water runoff. Water quality treatment facilities are the structural component of best management practices (BMPs); when used singly or in combination, WQ facilities reduce the potential for contamination of surface and/or ground waters.


Washington State Department of Ecology


An area inundated or saturated by ground or surface water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas (U.S. Army Corps of Engineers Regulation 33 CFR 328.3 (1988)).
Wetlands in King County include all area waterward from the wetland edge. Where the vegetation has been removed, a wetland shall be determined by the presence of hydric soils, as well as other documentation of the previous existence of wetland vegetation such as aerial photographs.

Wetpond sketch (PDF, 400-800kb)

Drainage facilities for water quality treatment that contain a permanent pool of water. They are designed to optimize water quality by providing long retention times (on the order of a week or more) to settle out particles of fine sediment to which pollutants such as heavy metals adsorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. For wetvaults, the permanent pool of water is covered by a lid which blocks sunlight from entering the facility, limiting light-dependent biologic activity.



Small, free swimming or floating animals in water, many are microscopic.