The 19 members of the Chongqing Green Volunteers Union saw massive belts of white foam floating on the Yangtze River everywhere they went, especially in the 45-kilometre-long Wu Gorge, the middle and most spectacular of the Three Gorges. Some of the bands of pollution stretched continuously for as much as two kilometres.

"I have never seen such an astonishing sight," said the group’s leader, biologist Wu Dengming, who in six other recent trips to the reservoir area had found nothing similar.

"It’s hard to count the number of sewage outlets along the river," he said. One of the largest the team saw disgorges a torrent of untreated wastewater at the confluence of the Yangtze and Wu rivers in Fuling.

"It looks like a yellow waterfall," Mr. Wu said. "And there is a terrible stench, even from far away." The wastewater contains poisonous substances, and will be an environmental time bomb for the reservoir area if allowed to continue to pour into the river, he said.

Another member of the team, environmental scientist Yuan Zhongxing of Chongqing University, said: "We really have no reason to be optimistic about the quality of the water after the reservoir is filled this June." The slow-moving river has already lost some of its self-cleansing capacity, he added.

Dr. Yuan said he was even more worried about the development pressure on wetland areas along the banks of the river. A strip of land on both sides of the Yangtze, all along the 660-km reservoir that will begin forming behind the dam in June, will be affected by the seasonal waxing and waning of the river, he said. This land should be left undeveloped, as a typical wetland ecosystem that will be crucial to the ecology of the reservoir area as a whole.

But some regions are making plans to develop the wetland belt, to replace good farmland that will be lost to the reservoir.

"This is such a dangerous idea," Dr. Yuan said. Developing this land will degrade an already fragile ecosystem, he warned. Reduce the filtering function of this natural ecological barrier, and more dangerous pollutants will flow directly into the reservoir water.

The green team also saw new garbage mounds appearing in many towns along the river that had already removed their old rubbish dumps as part of the reservoir cleanup campaign. In Fengjie and Wushan counties, for example, local residents continued to dump their garbage outside their homes in keeping with tradition, and fresh accumulations of rubbish can be seen along the banks of the river. Kitchen waste, styrofoam containers and plastic bottles are still thrown overboard from Yangtze passenger ships, they said.

The team is now preparing a report on its findings to be submitted to the Chongqing municipal government. The activists hope their efforts will help raise environmental awareness among local people – and raise the alarm with the authorities, that much more needs to be done to protect the water quality of the future reservoir.