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. If the temperatures increase the right foods will not grow and smolts will not have feed. These are just a few of the main problems the salmon face due to dam building. Society has cut the Columbia River in pieces and then manipulated each pieces for its own benefit. The Columbia River is made of multiple components but without each piece the river can not survive.