Before dams backed up the Snake, smolts coming
down from Idaho got to the sea in six to twenty days; now it
takes from sixty to ninety days, and few of the young salmon
reach salt water in time. The emasculated current is the single
largest reason that the number of wild adult salmon migrating
up the Snake each year has crashed from predevelopment runs
of 100,000 - 200,000 to what was projected to be 150-175 this
year.
http://www.theatlantic.com/politics/environ/dams.htm