Hydroelectric power is usually considered 'clean'. Generated from dams along rivers, there are no billowing smokestacks of black smoke to mark damage to the environment. Nor must we fear nuclear radiation from Chernobyl or Three-mile Island type accidents. But dams do harm the environment. This has been recognized in the United States where, since 1986, the Federal Energy Regulatory Commission gives equal weight to environmental and energy concerns when licensing or relicensing hydroelectric power facilities.

The reason is that dams change rivers. Species may not be able to adapt to the changed habitat. Older dams especially make it difficult for fish to move to their natural/optimal spawning grounds. Dams also increase water temperatures, decrease oxygen levels, and prevent the natural flow of silt, debris, and nutrients. The exotic-looking paddlefish, once found throughout the Mississippi River basin, is now threatened in the midwest.

Dams are built to regulate the flow of water and prevent floods, generate power, and reduce erosion. But just as forests occasionally need fires, rivers occasionally need floods. Controlled flooding of the Colorado River is being used to reverse damage to the Grand Canyon by dams.