QIUNATONG, China — 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 defense 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.