And they warn that tree roots, which make up about half the material they have been collecting, remain a serious hazard to ships on the river and could pose a danger to the dam itself.

Liu Gujun, who is leading a garbage-collection effort in Wanzhou, 300 kilometres upstream of the Three Gorges dam, told the Chongqing Morning Post (Chongqing chenbao) that his team gathered about 1,000 tonnes of waste last month. In a single day (Oct. 30), they hauled in 50 tonnes of floating vegetation and domestic garbage, several times the usual amount taken from the reservoir before it was raised recently at short notice from 135 to 139 metres above sea level.

Nobody expected such a serious situation to develop, said Ren Xiaojun, an official with the Wanzhou Urban Management Bureau. He said it was a shame that the bed of the reservoir had not been cleaned up more thoroughly before being filled to the 135-metre level in June.

"We didn’t dig out and remove the roots when we cut the trees down, because we thought the roots would just rot away after being submerged in the reservoir," he said.

The threat to navigation is just part of the problem, Mr. Ren said. He expressed concern about the safety of the Three Gorges dam, saying "the consequences are unimaginable" if a large number of tree roots collect at the dam site and start interfering with the operation of the turbines.

The floating roots become more dangerous as they become waterlogged after a long period of immersion. Mr. Liu said that while some of the root systems were small, others weighed more than a tonne. One day, he and his team were puzzled to see a "monster" suddenly appear, then disappear into the river. When they approached the creature, they saw that it was actually a giant tree root.

Mr. Liu said the roots, which are often hidden below the surface of the reservoir, cause frequent shipping problems. Vessels routinely sustain propeller damage as they pass through the Wanzhou section of the Yangtze. Mr. Liu’s own boat has struck tree roots six times since he began collecting garbage on the river in late September.

This summer, the propeller of a high-speed passenger hovercraft hit a tree root in the Wushan area, which threw the vessel off-balance and nearly caused it to crash into the rocky shore.

Pilots can have difficulty spotting the troublesome roots, which often blend in to the muddy river. A ship’s crew may be unaware of the problem until the propeller stops turning.

An estimated US$1 million is needed to address the floating pollution problem in the Wanzhou area alone, but officials are still negotiating the exact budget for the operation.

Mr. Liu and his team have not yet been paid for their work, the Chongqing Morning Post reported. To start the work without delay, Mr. Liu simply plunged into it, and persuaded nine other boat owners to join him.

In addition to contributing his own boat, Mr. Liu advanced US$6,000 from his savings to get the operation off the ground, the newspaper said. He will be reimbursed once the budget is finally approved.

"Why did I do that? Because I love this river. Because I have worked on this river for so many years," Mr. Liu said. "I can’t sit back any longer and watch the situation get worse and worse. I have to do something about it."

A recent survey conducted on both banks of a 70-kilometre stretch of the reservoir in Wanzhou district found that many other tree roots are very close to being submerged. With the water level scheduled to rise to 156 metres in 2005, many more roots are set to be washed into the reservoir along with all manner of other waste.

"Collecting garbage in the reservoir is not only tedious and onerous, but also a long-term battle," Mr. Liu said.