National Geographic : 2011 Mar
A DAMMED WORLD Hoover Dam and Lake Mead, Nevada MITCH EPSTEIN Dams tame floods, water crops (and people), and generate 16 percent of the world's electricity, carbon free. They have also displaced 40 to 80 million people and destroyed river ecosys- tems. More than half the world's large rivers are now dammed. Some, like the Colorado, are tapped out. Persistent drought has left a "bathtub ring" in Lake Mead, which supplies water to much of the Southwest.