Grazing cattle can trample the stream banks, their muddy footprints releasing even more silt. Pesticides applied to upstream crops can drain into a creek and poison fish. A road built alongside a stream can change the way water runs off. The stream is more apt to flood after a big rain. Where once there were trees and shade, the sun hits the water directly.
Direct sunlight can warm the water more than salmon eggs can stand.
Careless logging can ruin a salmon spawning stream. Branches and debris can block fish movements. Logs dragged along a slope, or a log truck crossing the smallest trickle, can churn up more silt. Of course logging removes trees and shade. Some of the worst damage was done before people quite understood how shade and plants are important to salmon.