And, from the 1930s, dams have made migration more difficult for both adults and smolts. As human impact continues to alter the salmon's spawning grounds, migration waters, and feedings areas, their numbers continue to decline. Generally, less than 1 percent of young salmon survive to return and spawn. L oss of Spawning Grounds G ood salmon spawning grounds with deep gravel and clean, cold, well-oxygenated water are becoming scarce. Erosion from clear cutting, fires, grazing, and road construction have caused silt to smother nests and eggs.
Nests also become vulnerable to freezing when water diversions and drought lower water levels.
Stream Life Challenges F ish face many hazards throughout their stream life, both as developing juveniles and migrating smolts or adults. A combination of any or all of the factors described below can be lethal to the salmon. This lack of oxygen can kill salmon fry and weaken smolts and adults. Water temperatures are further affected when the lack of streamside vegetation, clear cutting, and livestock use depletes shaded streamside areas. Subsequently, these increased water temperatures lower salmon's resistance to disease.