Surface Passage Structures in Dams

Spillways can be the safest route for juvenile fish to pass a dam, but the spillway may be difficult for the fish to find. Most salmon and steelhead juveniles like to stay in the top 20 feet of the water column. Spillway openings may be as much as 60 feet below the surface, too deep for the fish to use.

Beginning in 2001, some of the dams on the Columbia and Snake Rivers have been equipped with spillway weirs. These are movable gates on the upriver side. When juvenile fish are migrating, water flows over the top of the weir and out through the spill gate. Since fish do not have to dive down to find the spillway entrance, more fish are passed with less water spilled. And since the fish quickly find passage, they also may escape predators above the dam.