Downed trees, logjams and even beaver dams in streams are one way that nature helps provide good habitat for fish. Large wood helps create what biologists call habitat complexity.
A complex stream habitat includes several environments – fallen trees, large boulders, gravel, and vegetation -- often complemented by ground water springs. These help create pools, floodplains and shade, cooling water temperatures and offering young fish places to hide from predators as they grow or rest on their way to the ocean.
Years of human development, though, has left many streambeds in the Columbia River Basin scoured. This is why many projects to improve habitat for fish add wood to streams.