Each year from June to September the monsoon season strikes Central China. The massive rainfall floods the great Yangtze River basin, home to millions of people.

In 1998, in some of the worst flooding in recent years, more than 2,000 people died and almost 14 million were left homeless. By the end of the rainy season, the rising waters had impacted some 240 million Chinese, close to the population of the entire United States. China needed a solution to this costly problem.

As far back as 1919 Sun Yat-sen, the father of modern China, envisioned a dam that would control this chronic flooding, as well as harness the power of the historically unpredictable 3,700 mile-long Yangtze River, the third-longest river in the world. But it was only in the 1980s that Chinese leaders seriously proposed such a large project.

Construction on the Three Gorges Dam began in 1994.

The dam is located 1,000 miles west of Shanghai, China's largest city and a principal seaport. At this location, the river runs through three spectacular gorges -- narrow, steep canyons -- at the foot of the western Wu Shan Mountains.

The dam will be the world's largest, spanning 1.4 miles long and 630 feet high. It will produce a giant lake or reservoir more than 350 miles long and able to hold over one trillion gallons of water.

A great nation moving forward

The project, and its more than $24 billion price tag, has been controversial since its inception.

Chinese leaders claim that the dam will generate income for China's interior provinces through the sale of electricity and the ability of large 10,000-ton ocean vessels to travel a thousand miles inland to the city of Chongqing. Chinese leaders see the dam as a key to the modernization of China and as a symbol of the power of China.

"Successful completion of water storage means the Three Gorges Project has overcome the first challenge of nature," chief engineer Zhang Chaoran told the state-owned Xinhua News Agency.

Environmental and social impact

Critics claim that a much smaller project could have accomplished the same goals at a much smaller cost to the environment and people living in the region.

Creating the dam's reservoir will swallow towns, villages and farmland as well as ancient temples, burial grounds and other historic sites. Spectacular canyons, visited by tourists from around the world, will also end up under water.

So far, more than 700,000 residents of the flood path have been resettled and a total of 1.2 million will need new homes by 2009, when the reservoir is filled. Many of those resettled feel cheated by a government that they say promised relocation would improve their lives. More than half of those resettled are farmers.

"Instead of fields, we were offered wasteland to farm," a relocated peasant told The Christian Science Monitor.

Environmentalists fear that the dam will create a giant cesspool, filling up with sediment from the deforested slopes of Tibet's mountains. Also, factories along the Yangtze continue to dump poisonous wastes directly into the river, which, now dammed, is unable to flush the toxins away.

"Decades of accumulated trash from villages, hospitals, and cemeteries, including highly toxic waste material from factories and the corpses of millions of poisoned rats, are all still there," environmentalist Dai Qing told the Christian Science Monitor.

Dam opens to some shipping

On Monday the area of the Yangtze River affected by the dam opened to shipping in the first passenger test run of the world's largest hydroelectric project. Shipping had been closed on the river since April 10.

"China is opening a critical valve, linking the economic artery of the nation's west with the east,'' said Communications Minister Zhang Chunxian.

By Anne Schleicher, NewsHour Extra