IN 1997, THE SIERRA CLUB proposed to Congress that the 710-foot-high Glen Canyon Dam, which backed up the Colorado River, flooded Glen Canyon, and created Lake Powell when it was completed in 1963, be demolished. One congressman called the proposal a "bizarre idea," but it is not as far-fetched as it might seem.

In the twentieth century, more than six hundred dams were built with federal funds, mostly in the 1930s, bringing water to the deserts for agricultural development and fueling the growth of Las Vegas, Phoenix, and other desert cities. But the dams destroyed rivers and wildlife habitat, sometimes at a cost outweighing their economic benefits.

A number of dams, whose original purposes had become obsolete, have been demolished in recent years, as people recognize that wild free-flowing rivers prevent natural flooding, keep ecosystems healthy, and allow salmon and...