The most significant effect from the dams was the loss of salmon along the Columbia River. After two years they reach maturity then they migrate back to the river where they were born. Once in the rivers they spawn and then die. The young smolts are the only fish that return to the sea. Building dams along the Columbia has effected the salmon migration.
Dams are too tall for the fish to leap over.
Even if fishruns are present fish die as the leap over the dam. Fish numbers decline because fish can not make it back to spawning grounds to produce the next generation. Even if fish spawn the habitat along the river might not be suitable for their survival. Predators can easily catch the young smolts in slow moving water. The water temperature can be higher due to the presence of dams.