Clinging improbably to the alpine peaks are mist-shrouded villages whose residents are among the area’s dozen or so indigenous tribes, most with their own languages. “The project will be good for the local government, but it will be a disaster for the local residents,” said Wan Li, 42, who in 2003 left behind his big-city life as an accountant in the provincial capital, Kunming, to open a youth hostel here. “They will lose their culture, their traditions and their livelihood, and we will be left with a placid, lifeless reservoir.”

As one of two major rivers in China still unimpeded by dams, the Nu has a fiercely devoted following among environmentalists who have grown despondent over the destruction of many of China’s waterways. The Ministry of Water Resources released a survey in March saying that 23,000 rivers had disappeared entirely and many of the nation’s most storied rivers had become degraded by pollution. The mouth of the Yellow River is little more than an effluent-fouled trickle, and the once-mighty Yangtze has been tamed by the Three Gorges Dam, a $25 billion project that displaced 1.4 million people.

For many advocates, the Nu has become something of a last stand. “Why can’t China have just one river that isn’t destroyed by humans?” asked Wang Yongchen, a well-known environmentalist in Beijing who has visited the area a dozen times in recent years.

Image Four dams are proposed in Yunnan. Work on one in Tibet has begun. Credit... The New York Times

Opponents say it is no coincidence that the project was revived shortly before the retirement of Mr. Wen, a populist whose decision to halt construction was hailed as a landmark victory for the nation’s fledgling environmental movement. Although he did not kill the project, Mr. Wen, a trained geologist, vowed it would not proceed without an exhaustive environmental impact assessment.

No such assessment has been released. Given the government’s goal of generating 15 percent of the nation’s electricity from non-fossil fuel by 2020, few expect environmental concerns to slow the project, even if the original plan of 13 dams on the Nu has for now been scaled back to 5. “Building a dam is about managing conflicts between man and nature, but without a scientific understanding of this project, it can only lead to calamity,” said Yang Yong, a geologist and an environmentalist.

Some experts say that China has little choice but to move forward with dams on the Nu, given the nation’s voracious power needs and an overreliance on coal that has contributed to record levels of smog in Beijing and other northern cities. Still, many environmentalists reject the government’s assertion that hydropower is “green energy,” noting that reservoirs created by dams swallow vast amounts of forest and field.