Guide to San Francisco Bay Area Creeks

Creek & Watershed
Terms & Definitions

This page is designed to help you understand terms commonly associated with creeks and watersheds.

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The building up, or raising in elevation of the bottoms of water bodies, including streambeds and floodplains, through the deposition of sediment.


Alluvium n.
Alluvial adj.

Sediment deposits that are transported by streams and collected in riverbeds, floodplains, lakes, fans, and estuaries.



A life-history of animals that spend most of their lives in ocean water, but return to fresh water to spawn, including Pacific salmon and steelhead.



An underground body of fractured rock, sand, or gravel functioning as a reservoir or conduit of groundwater.


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Base level

An elevation in a stream channel that is fixed over the long term by being relatively unerodable, such as a dam, or not subject to erosion, such as sea level.


Bed load

Sediment, rocks and other materials that are transported by rolling or bouncing along the bottom of a stream.



Referring to the bottom of a body of water.



Best Management Practice


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The bed and banks through which a stream flows.



Realigning a stream by straightening it or dredging a new channel.



Where two or more streams meet and begin flowing together.



A naturally flowing stream that is usually considered smaller than and often tributary to a river.


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The lowering in elevation of streambeds or floodplains by the removal of bed load or bed rock.



Organic material resulting from decomposing plant and animal remains.



The volume of water flowing through a stream at a given place and time.


Dynamic equilibrium

A circumstance of a system which tends to be self maintaining due to the balance of cause and effect relationships that govern it. The sediment load of a stream is in dynamic equilibrium with sediment deposited on the flood plain by the competing forces of erosion and deposition.


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A circular current of water flowing opposite to the direction of the main stream.



The degree to which large particles, including boulders, rubble or gravel, are buried in fine sediments.



To wear away by the action of wind, water or other forces.



An enclosed body of water where river water mixes with salty ocean water.



The accumulation of plant nutrients (fertilizer) such as nitrogen and phosphorus components in a habitat or ecosystem. Up to a point, eutrophication increases biological productivity of a system, but eutrophication can easily overwhelm the ability of the system to process the plant growth and subsequent decay, causing cloudy waters and lowering of critical oxygen levels. A raw sewage spill is an extreme example of eutrophication, but runoff of garden fertilizers to streams and lakes can be plenty damaging.


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Flat lowland area that borders a stream and is covered by its water when the stream overflows. The floodplain is typically built up by alluvial sediments deposited by the stream.



Sudden rise or overflowing of a stream due to rainfall or snow melt.


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The geological study of the nature and origin of landforms, including the active forces of water, ice, wind, and gravity from which they derive their shape.



A smooth and gentle flow of water with little or no surface turbulence.



A regular increasing or decreasing in vertical elevation of the water surface of a flowing stream.



Water occurring below the Earth's surface that can supply wells and springs or gradually evaporate, or seep away.


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Incised channel

A channel cut into the surrounding terrain through degradation of a stream.


Large woody debris

A large piece of woody material in a stream channel whose diameter is greater than ten centimeters and length is greater than one meter. A very beneficial determinate of fish-habitat quality.


Limiting factor

Environmental factors, existing at adverse levels, that prevent organisms from reaching full biotic potential. It is typically used to mean that which limits population sizes of species in particular habitats. It is often the scarcest necessary resource, or most dangerous predator.


Longitudinal profile

A graph of the elevation over the length of a stream.


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Repeated, sinuous curving pattern of a river channel as seen from above, or the geomorphic evolution of such a pattern.



Actions taken to offset or prevent adverse effects on the quality of the environment.


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Nick point

A point of rapid erosion and incision at which a streambed is eroded to a new base level by a stream. A waterfall is a nick point.


source pollution

Pollutants from a broad area that enter the environment in small, diffuse flows. Contrasts with pollution from an outfall or discharge point.


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A looping river bend that is cut off from the main river flow by processes of meandering, typically becoming a lake.


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Point source pollution

Pollutants from a specific source or site that enter the environment, as in a discharge from an outfall or other discharge point.


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Spawning bed or nest in a streambed created by spawning trout or salmon for burying their eggs.



Rock, broken concrete, or other large durable materials layered to protect a stream bank from erosion.



Shallow rapids with surface turbulence resulting from water flowing swiftly over or around obstructions.



Connected to or immediately adjacent to the bank of a watercourse such as a river or stream.


Rip rap

Rocks that are broken and used to build or reinforce stream banks.



Water from rainfall or snow melt that ultimately reaches streams by flowing over the ground surface.


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Usually refers specifically to Pacific salmon and trout but also includes other fish in the Salmonidae family such as char, whitefish and related species.



When waves and currents erode stream banks or beds.



Inorganic and organic particulate matter that is carried in a stream, be it silt or boulders.



The process whereby sediment settles to the bottom of a streambed or is deposited on the floodplain.



Dead-end channels of slow-flowing or stagnant water in low marshy ground.



The production or depositing of eggs or sperm by aquatic animals.


Spawning habitat

Gravel beds or other parts of a stream or lake which provide suitable areas for fish to spawn.



Anadromous rainbow trout--spends much of its life in the ocean but returns to freshwater to spawn.


Storm drain

The opening into a culvert system that carries storm-water to a discharge point in a stream or ocean.


Stream bank

The sides of a channel that restrict lateral movement at normal water levels.



The bottom of a stream over which water flows.



Material made of mineral and/or organic matter that forms the streambed.


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Related to or part of land.



The path of a line connecting the lowest points of cross-sections along a streambed.



A stream or river that flows into another.



Disturbed or chaotic flow pattern of water or other fluids.



Opacity of water caused by suspended sediment or organic material.


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The entire area of land that drains into a particular channel or water body.



Areas such as swamps, marshes and bogs that contain permanent or temporary standing water.