To tame a river.

It will be twice as the Statue of Liberty and as long as 30 jumbo jets parked end to end. Water will gush through its turbines to generate power for millions of people.

It's the world's largest dam and it's under construction on the Yangtze River in China. If you happen to be on the Moon in 2009, when the dam is completed, you'll be able to see it through a telescope. (The only other human-built structure you'd see is China's Great Wall.)

The purpose of this colossal construction project? To tame the roiling Yangtze River, the fourth-longest river on the planet. The project's designers claim the Three Gorges Dam will prevent catastrophic floods and generate electric power for as many as one-ninth to one-third of China's 1.2 billion people. And by blocking the river's flow, the dam will form a new, massive lake-like reservoir open to shipping and tourism.

But people opposing the darn argue that the new lake will flood cities and villages, forcing more than 1 million people to move to higher ground. The rising water, they add, will threaten wildlife and permanently alter China's spectacular Three Gorges, the steep-walled canyons for which the dam is named.

If you think these arguments only take place thousands of miles away, think again. More than 63,000 dams have been built in the United States since 1900. Many were or still are controversial.

Do you think dams should be built? Read opinions on both sides, then debate and decide.

WATER POWER

The Three Gorges Dam could be a "clean" solution to China's energy problems. Right now. China gets about 75 percent of its energy from coal. But burning coal spews tons of harmful pollutants into the air. Sulfur dioxide, for instance, reacts with water vapor in the atmosphere to form acid rain; carbon dioxide, a "greenhouse" gas, traps Earth's heat.

Dams like the Three Gorges can help China convert to cleaner hydroelectric power; says Eugene Chang, an engineer at Harza Engineering, a Chicago firm that consulted on the project. Hydropower converts the energy of rushing water to electricity (see "Tech Connection," p. 18). A year's supply of the dam's hydropower could replace 50 million tons of coal. That's about 4 percent of China's annual supply -- enough coal to full a train stretching from New York to San Francisco.

And unlike coal, oil, and natural gas, says Hans Hasen, a consultant at Harza, the Yangtze's water is a renewable resource. Nature will replenish the flowing water as long as rain falls on the river. "We'll be out of oil in 40 years, out of coal in 60 years, and out of natural gas in 200 years," Hasen says. "But water is here forever."

The "superdam" could also help prevent flooding in central and eastern China. Heavy rains known as monsoons cause the Yangtze to' overflow its banks about once every 50 years. The dam would hold back flood waters by trapping some rain water in a new reservoir -- three Gorges Lake (see map, above).

Proponents say the dam will also create jobs. Currently, 40,000 workers are laying the foundation. Eventually, they will build shipping lanes so that massive ships can navigate the river from the coastal city of Shanghai into Three Gorges lake. There, shipping companies and other industries will set up shop.

DOWN WITH THE DAM

But new industries could mean the downfall of farmers living near the dam. The dam's huge structure will slow the river's normal flow of approximately 300 billion gallons of water per day. Backed up behind the dam, the river will overflow its banks, flooding 62,000 acres of family and to create the reservoir.

The rising water will completely submerge 13 riverside cities, 140 large towns, and numerous small villages. Some 1.2 million people will be forced to relocate to less fertile areas on higher ground, says Patrick McCully, campaign director at International Rivers Network, a Berkeley, Calif., environmental and human-rights group. The government has already forced 20,000 to 40,000 people to move.

Says Wang Cheng Liang, a farmer whose land win be flooded when the darn is built, "I don't know how we can make a living up there [on higher ground]. It's all stones."

McCufly and others also contend that hydropower isn't as clean as it seems. "The power plant isn't belching smoke," he admits. "But in fact, hydroelectric dams destroy ecosystems."

Fish and other species that normally swim upriver to mate and lay eggs won't be able to bypass the dam, he explains. In addition, pollutants like mercury and arsenic, which used to flow out to sea, will now build up to potentially toxic levels in Three Gorges Lake. McCully fears that these concentrated pollutants will kill fish, reptiles, and other wildlife, including the endangered baiji river dolphin.

Higher water levels in the lake area will also damage bamboo groves and bottom-rooted aquatic plants, environmentalists say. That means giant pandas and Siberian white cranes, which feed on these plants, will suffer, too.

In addition as the water rises, the beauty of the Three Gorges canyons will be lost forever. "Flooding the Three Gorges would be like destroying the Grand Canyon," says Roger Schlickeisen an environmentalist.

FLOODS TO COME

Dams aren't even foolproof when it comes to flood control, McCully warns. And some experts say an earthquake could strike Three Gorges Dam with tragic consequences. "Dams create a false sense of security," McCully says. As a result, people living downstream may neglect to fortify embankments that have helped hold back floods for thousands of years.

A final drawback: Dams are expensive. Three Gorges will cost some $17 billion to $50 billion to build, says McCully. That money, he says, would be better spent fortifying smaller dams and exploring other energy sources, like solar and wind power. "We just can't go on destroying rivers and wildlife," he says.

What do you think? Should dams be built? Read the opinions of some of your peers, then debate and decide.