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Chapter 4 -- Watershed Processes and Aquatic Ecosystems (continued)

Size of Stream Substrates

As sediments are transported by the stream, they are sorted by size classes with zones of deposition for each size class. Particles too large to be moved by the flow of water remain in the channel and may be overlain by smaller particles to form the banks of the thalweg. As the discharge changes, the sorting and zonation of the particles shifts, causing some mixing of particle size classes. At low flows, only small particles can be moved and deposited. Consequently, at low flows there is a tendency for small particles to fill the interstitial spaces between the larger particles, reducing permeability. In stream channels with a high proportion of fine sediments, there is greater tendency for filling the interstitial spaces. This reduces the quality of spawning habitat for fish. Furthermore, embedding of stream gravel and cobbles with sand reduces the productivity of benthic macroinvertebrates, which reduces the food available to fish and potentially reduces total fish production.


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