River Ecology to Be Altered by Dam but Chinese Officials Say That Demands Must Be Placed on Nature to Serve the Needs of Society Series: TROUBLE ON THE YANGTZE RIVER. Last in a Series. Part 1 Appeared on July 18, Part 2 on July 22

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FROM deep inside a darkened tank streaked black with mildew, a sound floats upward like a soft whisper.

Inside, "Qiqi," (pronounced chi-chi), the only Yangtze River dolphin in captivity, exhales through his blowhole and waves his long beak, pacing his shallow home with sonar as he has done for 11 years.

To the few Chinese scientists trying to save the species from extinction, Qiqi's breath sounds more like a sigh every day.

China's leadership next month will consider building a dam on the Yangtze that would hasten the steady drift of the river dolphin toward extinction.

The Yangtze dolphin population has shrunk by a third since the International Union for the Conservation of Nature and Natural Resources declared it one of the world's 12 most endangered animals in 1986. Only about 200 of the silver cetaceans remain.

"If we don't take further steps, the dolphin will completely die out," says Cao Wenxuan, an ichthyologist at the Institute of Hydrobiology in Wuhan.

The threat to the dolphin is one example of how the world's largest dam would damage the environment in the world's most densely populated river basin.

Beijing will base its decision on building the dam on two studies claiming that gains in flood control, hydropower, and shipping would far outweigh the injuries to man and the environment caused by the dam, Chinese and foreign scientists say.

The studies, one conducted by China and the other by a consortium of Canadian companies in the mid-1980s, are flawed, the scientists say. According to them, the studies are too superficial to enable Beijing to make a valid environmental assessment.

Critics charge that the dam would harm the environment for man, the river dolphin, and other species in several ways. Fishing disrupted

Most notably, the dam would disrupt the rhythmic rise and fall of the river, causing losses for some 75 million people who eke out a living downstream from the dam site, the scientists say.

The losses to fishermen and farmers will be especially severe because China lacks the money to shepherd them into a comparable livelihood, says Joseph Larson at the University of Massachusetts, Amherst.

There is "no excuse" for Beijing's failure to "factor the losses for these people into the costs of the dam," says Dr. Larson, a professor of wildlife biology and director of the university's Environment Institute.

Moreover, the dam would likely curtail the river's capacity to flush out pollution and replenish nutrients downstream, observers say.

The gigantic structure would probably reduce the volume and quality of water for city dwellers and peasants, and increase soil erosion and sedimentation.

Finally, the dam could threaten an array of wildlife, including the Siberian crane, Chinese sturgeon, and the dolphin, according to the scientists. The possible changes to the environment would be so far-reaching and complex that they defy accurate prediction, they say.

"The scale of the Three Gorges project is so vast that there is no way that anyone will be able to figure out how to classify the social and ecological impact of it," says Baruch Boxer, head of the department of human ecology at Rutgers University.

The uncertainties highlight a paradox reminiscent of Frankenstein: Mankind's awesome power to control nature exceeds its ability to comprehend the myriad consequences resulting from the exercise of such power. …