Every year in pageants that are as ancient as they are majestic, recently spawned salmon, steelhead trout and other fish make their way down the Columbia River, on the Oregon-Washington state border. As they do, they attempt to run a sometimes lethal gauntlet of six to eight hydroelectric dams.

The massive structures, including the legendary Bonneville Dam outside Portland, Ore., have elaborate and labyrinthine fish bypass systems to help the creatures past the turbines. Nevertheless, at Bonneville as many as 45 percent of the fish go through the turbines in the summer. The enormous, propellerlike blades, which can reach 75 revolutions per minute, are too large and slow to pur¿e the fish. Rather they subject them to turbulence, rapid changes in hydrostatic pressure and strong shear forces. Of the creatures that go through Bonneville, up to 12 percent perish as a result of their injuries--or, more likely, because they are no match for predators in their weakened state.

   
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