China's New Megadam
The world's largest concrete dam--indeed, its largest concrete structure--spans the Yangtze River in south central China. The Three Gorges Dam, named for the steep, winding canyons upriver, should provide 10 percent of China's electricity. The dam also enables huge ships to move much farther up the river, and may help protect against floods downstream.
But the dam has created a huge reservoir--half as long as the state of California--causing the relocation of more than 1.2 million people. The destruction of river habitat is pushing several species of fish, birds, and other creatures towards extinction.
By the Numbers
Three Gorges Dam measures about 2.3 kilometers (1.5 miles) across
The dam should generate as much electricity as 18 nuclear power plants.
During the peak of construction, the Three Gorges Dam project employed 26,000 workers.
The Dam's Downsides
Aside from the problems the dam is causing for animals and people, critics have raised other concerns.
There's so much water in the Three Gorges reservoir that some geologists say its weight could set off earthquakes.
The dam's power-generating turbines don't pollute or contribute to global warming the way burning coal does. But as the trees and plants submerged by a reservoir decompose, they emit large amounts of methane and carbon dioxide--greenhouse gases that contribute to climate change.
Yangtze River water carries lots of sediment, which tends to build up behind the dam, filling the reservoir and reducing the dam's efficiency. As part of the solution, the Chinese government is building several other dams on upstream tributaries to trap sediment.
The dam takes its name from the three steep gorges upstream: Xiling, Wu, and Qutang. Treacherous rapids along the river, and the steepness of the surrounding terrain have kept this region relatively isolated for thousands of years.
Workers poured the last concrete for Three Gorges Dam in December 2004. When the reservoir is filled to capacity, water levels will be up to 110 meters (360 feet) higher than before the dam was built.
A large dam like Three Gorges dramatically alters wildlife habitats along the river. The dam transforms the cool, free-flowing waters upstream into a warm, artificial lake. Water released below the dam, carrying much less sediment, flows faster and tends to scour away sandbars and riverbanks. Even before the building of Three Gorges, several species were threatened by the river's other large dams.
The baiji, a river dolphin found only in the Yangtze, is teetering on the brink of extinction. Many baiji were accidentally impaled by fishhooks, and several earlier dams on the river blocked baiji migrations and destroyed much of their habitat. Efforts to breed the baiji in captivity were unsuccessful.
Chinese sturgeon were a common catch for fishermen until the 1980s. Today, they're almost impossible to find. The Gezhouba Dam, built just below Three Gorges, has hindered sturgeon from reaching their spawning grounds. A government-sponsored breeding program has made little progress in replenishing the population, and Three Gorges Dam may further imperil the species.
At Home in the Wetlands
Siberian cranes mate in Siberia, but nearly all of them fly south for the winter, to a lake along the shores of the Yangtze River. They depend on water pulsing into and out of this lake, and Three Gorges Dam may damage this habitat by interfering with water flows.
To make way for the world's largest manmade reservoir, the Chinese government moved over 1.2 million people from 13 cities and more than 1,000 villages and towns. Most were moved to higher elevations nearby, but some were sent to faraway provinces. The reservoir also covered rich farmland, which was scarce at higher elevations. So farmers, who made up almost half of the relocated people, either coped with smaller farms or found other ways to make a living.
By the Numbers
Three Gorges Reservoir submerged more than 30,000 hectares (74,000 acres) of cultivated farmland
Farewell, Old Zigui
The city of Zigui, once home to more than 30,000 people, was dynamited in March 2002 to make way for the Three Gorges Reservoir that now stretches hundreds of miles upriver. The city's residents were moved to a newly built city on higher ground.
The rising waters not only disrupted the lives of people along the river, they also submerged hundreds of historic landmarks and archaeological sites. Although Chinese scholars and local historians scrambled to save as many temples and relics as possible, they couldn't finish the work in time. In particular, many archeological sites of the Ba people, whose history dates back 4,000 years, were never fully explored.