BANGKOK — As China was stricken by the coronavirus in late February, its foreign minister addressed a concerned crowd in Laos, where farmers and fishers across the Mekong River region were contending with the worst drought in living memory.

His message: We feel your pain. The foreign minister, Wang Yi, said China was also suffering from arid conditions that were sucking water from one of the world’s most productive rivers.

But new research from American climatologists shows for the first time that China, where the headwaters of the Mekong spring forth from the Tibetan Plateau, was not experiencing the same hardship at all. Instead, Beijing’s engineers appear to have directly caused the record low water levels by limiting the river’s flow.

“The satellite data doesn’t lie, and there was plenty of water in the Tibetan Plateau, even as countries like Cambodia and Thailand were under extreme duress,” said Alan Basist, who co-wrote the report, which was released on Monday, for Eyes on Earth, a water resources monitor.

“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.”

As China’s geopolitical weight has grown, its leaders have cast the nation as a different kind of superpower, one that is concerned, as the Chinese phrasing goes, with “win-win” relationships with other nations.

But some countries, like Sri Lanka and Djibouti, have fallen into what critics fear are debt traps, as strategic projects end up in Chinese hands. Other African and Asian nations are worried that China is simply another imperial power eager to vacuum up natural resources without concern for the local populace.

“This is part of China’s business development,” said Chainarong Setthachua, a lecturer and Mekong expert at Mahasarakham University in northeastern Thailand. “The lay people who depend on the resources of the Mekong River for their livelihoods and income are automatically excluded.”

The data modeling created by Mr. Basist and his colleague Claude Williams measures the various components of a river’s flow, from snow and glacial melt to rainfall and soil moisture. The scientists found that for most years, the natural, unimpeded flow of the upstream Mekong roughly tracked water levels measured downstream at a gauge in Thailand, with occasional exceptions when dam reservoirs in China were being filled or released.

When there was a seasonal drought in China, the five downstream nations — Myanmar, Thailand, Laos, Cambodia and Vietnam — would eventually feel it. When there was overabundant water in China, floods ensued in the Mekong basin.