Effects of Coarse Granitic Sediment on the Distribution and Abundance of Salmonids in the Central Idaho Batholith

The effects of coarse granitic sediment on the abundance, distribution, growth, and behavior of juvenile salmonids and the abundance of aquatic insect drift were assessed by adding sediment to artificial and natural stream channels . These data were compared to two natural streams in the central Idaho batholith which already contained large amounts of sediment by correlating fish and insect drift abundance with percentage riffle sediment, riffle size, percentage cover in the pools, and average pool substrate imbeddedness. Densities of juvenile steelhead trout (Salmo gairdneri Richardson), chinook salmon (Oncorhynchus tshawytscha (Walbauum)), and cutthroat trout (Salmo clarki Richardson) in the artificial stream channels decreased as we increased the amount of sediment in the channels. The densities of fish remaining in the channels during the winter tests (water temperature below 5 C) were smaller than in the summer tests (water temperature above 5 C) . Total insect drift density was not affected by the addition . of sediment to the riffles in the artificial streams, but evidence of a community shift to Diptera was observed. Addition of sediment into Knapp Creek reduced fish densities due to loss of cover and pool volume. Insect benthos and drift abundance was not significantly changed. Fish density correlated best with cover and insect drift density in the natural streams studied. Sediment altered the amount and quality of cover in the pools studied. Insect drift density correlated best with riffle size, and to a lesser degree with percentage sediment in the riffles