Dams and hydropower operations have modified the level, timing, frequency and duration of stream flows. They have blocked the movement of fish both upstream and downstream, dewatered stream segments below dams, caused loss of upstream habitat, and increased predation in reservoirs. Smolts and juvenile fish migrating downstream through the reservoirs encounter slower moving water, increasing the time it takes for them to reach the ocean, thereby increasing their chances of dying from predation and diseases. In addition, the absence or inadequacy of fish ladders or other by-pass systems block or limit adult migration upstream, closing off many miles of potential spawning and rearing habitat. Agricultural Practices Agricultural practices that may adversely affect salmon include diking, draining, filling, stream channelization, removal of large woody debris, installation of riprap along stream banks, removal of riparian vegetation, road building, diversion of surface and ground water for irrigation and agricultural processing, and pesticides and fertilizer applications.
Irrigated agriculture requires diversion of water, which reduces streamflows.
In some years this leaves little or no water for salmon and other aquatic species. Return flows, while perhaps increasing the amount of water in streams, degrade the water quality by raising its temperature and adding dissolved chemicals. Unscreened or improperly screened diversions can have devastating effects on juvenile fish. Forest Practices Forest management activities such as road building, timber harvest near streams or on steep or unstable areas and the application of chemicals have damaged fish habitat and water quality. These forest practices also resulted in loss or degradation of spawning and rearing habitats, contributing to the listing of some salmon runs.