“There was just a huge volume of water that was being held back in China,” Mr. Basist added.

The Mekong is one of the most fertile rivers on earth, nurturing tens of millions of people with its nutrient rich waters and fisheries. But a series of dams, mostly in China, have robbed the river’s riches.

Those who depend on its inland fisheries say their catches have declined precipitously. Persistent droughts and sudden floods have buffeted farmers.

Beijing’s control of the upstream Mekong, which provides as much as 70 percent of the downstream water in the dry season, has raised hackles, even though the Southeast Asian nations depend on trade with China. While the Chinese government has introduced a global development program that it says will benefit poorer trading partners, a backlash is growing among countries that feel they are losing out.

“The problem is that the Chinese elite see water as something for their use, not as a shared commodity,” said Brian Eyler, director of the Stimson Center’s Southeast Asia program and author of “Last Days of the Mighty Mekong.”