QIUNATONG (China), June 19 — Three great rivers rush through parallel canyons in the mountains of southwest China on their way to the coastal plains of Asia. At least 10 dams have been built on two of them, the Mekong and the Yangtze. The third remains wild: the remote, raging Nu, known as the Salween in Myanmar, where it empties into the Andaman Sea.

No dam stands in the path of its turquoise waters. It is the last free-flowing river in China.

Environmentalists have waged a passionate defence of the Nu for more than a decade, battling state hydropower firms determined to build dams to harness the river, whose name in Chinese means “angry.” It is an epic struggle that has veered from victory to defeat and back again several times and has recently taken on new significance:

With global temperatures rising, can China afford to protect its rivers and forgo an alternative to the coal-fired plants responsible for much of the world’s greenhouse gas emissions?

Green advocates across the country have argued that dams on the Nu would force the relocation of tens of thousands of people, destroy spawning grounds for fish and threaten the livelihoods of farmers and fishermen downstream, especially across the border in Myanmar and Thailand.

But China, the world’s biggest emitter of greenhouse gases, has promised to begin scaling back its use of coal even as its power needs continue to rise. The government pledged that one-fifth of the nation’s energy use will come from non-fossil sources by 2030, and it intends to reach that goal in large part by building more dams. Like the United States, China considers hydropower its biggest source of clean energy.

In the latest twist in the battle over the Nu, provincial officials told Chinese reporters in March that they would suspend construction of small dams and instead establish a national park along the river. But environmentalists remain wary.

I recently spent nearly a week along a 250-mile (402-km) stretch of the river, travelling from the prefecture capital of Liuku to the Tibetan border to see what was at stake in this battle and why environmental groups and local residents have fought the dams so fiercely.

Garbed in traditional costume, Yu Wulin checks his smartphone in the main room of his guesthouse in Laomudeng, a village in China’s Nu River valley, March 30, 2016.

This valley, in China’s southwest Yunnan province, is home to a kaleidoscope of ethnic and religious communities unparalleled in China. Farmers tend to fields along steep, terraced valley walls. Pastors hold services in churches overlooking the waters, while Tibetan lamas perform rituals in hilltop monasteries. And guesthouse owners open their doors to tourists eager to glimpse this wild corner of their country.

Dams would result in the flooding of many of these villages and undermine the river’s role as a lifeline through the valley.

Over the years, government agencies and state firms have proposed building multiple dams along the 1,700-mile river. Most of these plans have been scrapped, and the fight is now focused on proposals for four dams on the pristine upper stretch that flows through this valley, and a fifth one in Tibet.

These dams would contribute to the 350 gigawatts of hydropower capacity that Beijing wants to have built by 2020. By comparison, the United States has more than 80 gigawatts of hydropower capacity.

Preliminary construction work has been touch and go for years, and the four sites were abandoned when I visited. There were suspension bridges, rusted transport skiffs and small man-made caves in the valley walls, but no workers in sight.

“The dam is no good for the environment, so the government stopped it,” said Abao, a resident of Dongfeng Village, far up the valley. He recently visited the fifth site, upriver in Tibet, and said construction appeared to have halted there.

But given past swings, residents said they were worried about work resuming.

“The water flow wouldn’t be normal, and there would be less fish in the water,” said Yang Wendong, 37, an ethnic Nu resident of a mushroom-rich mountain village, Qiunatong, by the Tibet border. “The benefits from hydropower are temporary.”

In Xiaoshaba, near the proposed dam site farthest downriver, the government has already relocated residents, the start of what will be a huge resettlement effort along the entire valley if the dams are approved.

The village’s 120 households have been forced to give up their livestock and farmland and move into two-story buildings nearby. Some run small shops or lease space in the buildings.

A road being built alongside the Nu River which facilitate both tourism and the construction of more dams, in China’s Yunnan province near the border with Myanmar, March 31, 2016.

As I stood on a road overlooking the empty village, Yu Wenxiang, 60, invited me into his sister-in-law’s house and complained that the government had yet to compensate his family with a new home. “The government promised there were a total of 10 homes they would still give out, but they haven’t handed those out,” he said. “It has been a long time.”

To the north, in the village of Laomudeng, Yu Wulin, a guesthouse owner, said the dams would have “no good benefits for the local people,” and would choke off the trickle of tourists coming to the region since 2003. That was when it was recognised as one of the world’s most ecologically diverse and fragile places and designated a Unesco World Heritage Site.

In March, officials from Yunnan province — who along with the China Huadian Corp, one of the country’s largest state energy firms, had promoted the dams — appeared to reverse themselves and propose a national park.

Residents I spoke with applauded the idea. “My generation will not see the benefits of the national park project, but the next generation will,” said Yang Yi, a minivan driver in Gongshan, an upriver town.

But Wang Yongchen, director of Green Earth Volunteers, a group in Beijing that has led the fight against dams on the Nu for more than a decade, expressed some scepticism. She said the officials’ statements could be interpreted to mean only that construction of smaller hydropower stations using water from the Nu’s tributaries would be suspended. More than 100 of these smaller stations have already been built in the valley.

“We want the central government to make a decision on these four dams, to cancel them,” Wang said.

Wang and other environmentalists acknowledge that China needs to wean itself off coal and other fossil fuels to combat global warming, but maintain that it can do so without resorting to as much hydropower or nuclear power as proposed by the government.

Some said the construction of dams could contribute to climate change because organic matter submerged by dams and reservoirs may release significant amounts of methane, a greenhouse gas, when they decompose.

There are also commercial rationales for not building the dams. Some energy analysts say much of the electricity generated by hydropower in Yunnan is wasted because the transmission system to the national electricity grid is limited and outdated. Others say the electricity does not end up on the grid because the network gives coal-fired plants preferential treatment over firms specialising in alternative power generation.

“Yunnan already has more hydropower capacity than it uses,” said Zhang Boting, vice chairman of the China Society for Hydropower Engineering. “Many stations in Yunnan are already letting water run free because there is no market for it.”

A farmer prepares a terrace field for planting rice in the Nu River valley in China’s Yunnan province, March 31, 2016.

A spree of dam building is already underway on other major rivers of western China, which flow into neighbouring countries and are critical for life across Asia. In October, China began operating the giant Zangmu Dam on the Yarlung Tsangpo in Tibet, upsetting India and Bangladesh because the river is an important waterway in their countries. Proposed dams on the Mekong have raised similar anxieties.

But environmentalists have succeeded in persuading the central government to block some projects. In Yunnan, the proposed Guonian Dam on the Mekong, known as the Lancang in China, was cancelled because of its potential impact on a glacier. Last year, Beijing also rejected plans to build a dam on the Yangtze because of the potential effects on a protected zone for fish.

Some climate change researchers say building hydropower capacity is critical for China as it tries to move away from coal.

“Hydropower, in China’s sustainable development process, occupies a rather important role,” said Yang Fuqiang of the Natural Resources Defence Council. “But with regards to the Nu River,” he added, “I personally think the best thing for China is to preserve a perfect, untouched river. Don’t construct large-scale dams on it.”

Chen Zuyu, a prominent hydropower scientist, estimates that the country has installed less than half of the 700 gigawatts of potential hydropower capacity, and said it should push ahead.

“China is still in the golden era of hydropower development,” he said. “China can keep developing hydropower for another 30 years, and then it will be done.”

Chen said the Nu was ready to be developed, but noted, “Environmental issues are political issues and are the hardest to resolve.” — The New York Times