http://en.wikipedia.org/wiki/Lotic_system_ecology
The inorganic substrate of lotic systems is composed of the geologic material present in the catchment that is eroded, transported, sorted, and deposited by the current. Inorganic substrates are classified by size on the Wentworth scale, which ranges from boulders, to pebbles, to gravel, to sand, and to silt.Typically, particle size decreases downstream with larger boulders and stones in more mountainous areas and sandy bottoms in lowland rivers. This is because the higher gradients of mountain streams facilitate a faster flow, moving smaller substrate materials further downstream for deposition.Substrate can also be organic and may include fine particles, autumn shed leaves, submerged wood, moss, and more evolved plants.Substrate deposition is not necessarily a permanent event, as it can be subject to large modifications during flooding events.
The inorganic substrate of lotic systems is composed of the geologic material present in the catchment that is eroded, transported, sorted, and deposited by the current. Inorganic substrates are classified by size on the Wentworth scale, which ranges from boulders, to pebbles, to gravel, to sand, and to silt.Typically, particle size decreases downstream with larger boulders and stones in more mountainous areas and sandy bottoms in lowland rivers. This is because the higher gradients of mountain streams facilitate a faster flow, moving smaller substrate materials further downstream for deposition.Substrate can also be organic and may include fine particles, autumn shed leaves, submerged wood, moss, and more evolved plants.Substrate deposition is not necessarily a permanent event, as it can be subject to large modifications during flooding events.
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