Big dam, big problem? (Earth/Engineering).

In November, China's Yangtze River changed forever. Earth's third-longest river once flowed freely over 6,300 kilometers (3,915 miles), from Tibet to the China Sea. Now its waters are plugged behind a wall of concrete and boulders--the foundation of the Three Gorges hydroelectric dam. When completed in 2009, it will be the world's largest dam, spanning 2.3 km (1.4 mi) and standing 185 meters (607 feet) high.

Why dam the Yangtze? The water locked behind the dam will flood a 600 km (375 mi)-long reservoir for irrigating crops; and water pouring through 26 hydroelectric turbines will generate 84.7 billion kilowatt (unit measuring electrical power) hours of electricity annually--enough to power tens of thousands of homes. "It's the equivalent of burning 50 million tons of coal a year," says Ellen Wohl, a Colorado State University geologist.

The downside: The reservoir will displace 1.3 million people. Sediment--silt and sand--trapped behind the dam will harbor industrial waste, impacting water quality. Downriver, the Yangtze, depleted of sediment, could flow more forcefully, erode its banks, and flood erratically. And changing the river will endanger at least 80 fish species. "With big river systems, you don't know how widespread the effects will be," says Wohl.