The Dam Dilemma

Dams give us hydroelectric power, flood control, irrigation, and drinking water for billions of people. China's Three Gorges Dam is the largest power station on Earth. The Hoover Dam tamed the Colorado and turned desert into farmland. As feats of engineering, dams are genuine marvels.

The trouble is that every dam comes with a hidden bill.

That bill is steep. Three Gorges forced 1.3 million people from their homes and drowned ancient towns under its reservoir.

Dams also trap the sediment that builds river deltas. The Nile Delta is now sinking and shrinking because the Aswan Dam holds back the silt that once rebuilt it. And by blocking the routes fish swim to spawn, dams have gutted salmon runs around the world.