The cycle of nature is both complex and simple. Salmon and steelhead hatch up stream, often hundreds of miles from the ocean. Surviving juveniles use the river current to migrate down stream. At some point during their journey, chemical changes in their bodies will allow the fish to enter the ocean. If they don't go to sea immediately after the change the fish will die.
Dams significantly slow the currents, their turbines also grind up many juveniles.
Perhaps five fish in a hundred which would have made it out to sea prior to the construction of the dams now do so. Those who do survive spend three or four years before they come home to spawn. They gather at the mouths of rivers where another chemical change which allows them to live in fresh water again occurs. Then the final struggle begins. The fish try to swim up river, where they will spawn and die.