For the last hundred years, people around the world have been building dams to control flooding and harness the water and energy coursing through large rivers. Today, dams generate 20 percent of the world's electric power, and provide needed water to cities and for irrigation.

But in taming the world's rivers, these massive marvels of engineering have blocked migration routes for many fishes and re-shaped rivers and other habitats. The reservoirs behind large dams have also displaced millions of people from their homes. And large dams often don't live up to expectations--most irrigation dams deliver less water and are less profitable than expected, and some hydropower dams generate less power than predicted. Usually, the bigger the dam, the greater the negative consequences.

Smarter solutions

Instead of building large hydropower dams, we could instead develop other power sources such as wind, solar, and tidal power.

Damming the World's Rivers

The majority of the world's large dams are in the U.S. or Europe. But Asia and South America are building the most dams today. Will future generations regret the social and environmental costs?

By the Numbers

More than 60 percent of the world's rivers have been dammed or diverted

Over the last 50 years, humans have built more than 47,000 large dams around the world

Nurek Dam, in Tajikistan, is the world's tallest: as tall as a building 65 stories high

Dam After Dam

In the northwestern U.S., more than 19 big dams block the Columbia River. So on a river 2,000 kilometers (1,240 miles) long, only about 70 kilometers (43 miles) flows freely.

La Plata River Basin

More than 40 dams have been proposed for the La Plata River basin, South America's second largest. The river supports dozens of bird and fish species found nowhere else.

Blue Danube

The Danube River basin, spread over 17 countries, is already fragmented by several dams. Eight new dams are planned, under construction or recently built.

Disputed Waters

The Nile River basin spans 10 African countries, but a 1959 treaty grants Egypt rights to use the largest share of Nile water. In recent years, some upstream countries have been challenging that treaty, arguing that they deserve a greater share of the Nile's water.