Dear EarthTalk: Many people oppose dams because they change the flow of rivers and affect the migrating patterns of fish and other species, but aren’t they also a great renewable energy source? -- Ryan Clark, Milton, WA
Hydroelectric dams are among the greenest and most affordable electricity sources in the world—and by far the most widely used renewable energy sources—but they also take a heavy environmental toll in the form of compromised landscapes, ecosystems and fisheries. Hydroelectric dams have been an important component of America’s energy mix since the powerful flow of rivers was first harnessed for industrial use in the 1880s. Today hydroelectric power accounts for seven percent of U.S. electricity generation—and some two-thirds of the country’s renewable power—according to the U.S. Geological Survey.