A severe drought that has caused water levels in Southeast Asia’s Mekong River to drop to their lowest in more than 100 years could have devastating consequences for fish, as well as the tens of millions of people living and working along the river, experts warn.

The crisis began when critical monsoon rains, which usually start in late May in the Mekong region, failed to arrive. Dry conditions, driven by the El Niño weather phenomenon and exacerbated by climate change, persisted well into July. At that time, observers say, the situation was made worse by hydropower dam operators upstream, in China and Laos, withholding water for their own purposes.

Although the rains finally began to fall in the last week in much of the river basin, with water levels now slowly rising, experts warn that the potential damage from the drought could be worse than in 2016, when another drought caused forest fires around Tonle Sap Lake in Cambodia and widespread disruptions to food production.

Many rice farmers in the region have been unable to plant their main crop, raising fears of a heavily diminished harvest this fall. Less water flow could also have a devastating impact on fish reproduction in the Mekong River basin. This is normally the time when fish use rising water levels as a cue to spawn and to disperse their young, but there is little evidence of this happening so far this year.

Perhaps even more alarming, experts expect that droughts and disruptions to the water flow of the Mekong will become more common, and they warn that it could eventually lead to the collapse of the entire ecosystem.

“With the completion of more mainstream dams and the cumulative effects of climate change, that tipping point” for when the Mekong can no longer sustain these changes “may be coming closer,” says Brian Eyler, the Southeast Asia program director at the Stimson Center in Washington, D.C.

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Flood Pulse

Originating in the Tibetan highlands, the Mekong River flows through six Asian countries, including China, Myanmar, Thailand, Laos, Cambodia, and Vietnam, before emptying into the South China Sea. The river basin is home to the largest inland fishery in the world and more than 60 million people depend on it for their livelihoods.

Few rivers in the world rise and fall with the seasons as much as the Mekong, which can drop up to 40 feet in some places at the end of the dry season. When the monsoon rains arrive, they normally produce a flood pulse that brings with it sediment essential to agriculture as well as enormous amounts of larvae and tiny fish, including many critically endangered species such as the Mekong giant catfish, that are swept into the Tonle Sap Lake and other floodplains where they can mature.

Every year, scientists have been collecting samples of these tiny fish and larvae on the Mekong River near Phnom Penh, Cambodia. However, so far this year, sub-normal water levels have produced no flood pulse, and the researchers have not seen any dispersal of fish larvae.

“Without the flood pulse, fish may delay or skip spawning,” says Zeb Hogan, a National Geographic Explorer and fish biologist at the University of Nevada, Reno, who leads a USAID project called Wonders of the Mekong. “For rare and endangered species, this situation threatens their survival, and for commercially important fish species, future harvests could be significantly reduced.”

According to Peng Bun Ngor, a fish ecologist with the Cambodian Fisheries Administration, the low river flow also forces brood fishes to concentrate in spaces where they become more vulnerable to being captured by fishers. “This adds to the existing problem of low recruitment,” he says.

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Dam Politics

This year, the dry conditions in the Mekong region persisted due to warm Pacific Ocean currents known as the El Niño effect. But climate change is also a driving factor, experts say, causing the monsoon season to shorten considerably.

“I have no doubt that this present drought is caused by the shift in world weather patterns as the result of global change, especially warming trends, and it would not be surprising if it lasted several more years,” says Peter Moyle, a biology professor emeritus at the University of California, Davis.

Moyle and others say dams on the upper parts of the Mekong are contributing to the degradation of the entire river system.

“Dams collect sediment, block fish migrations, and create reservoirs that support a fraction of the fisheries that the equivalent reach of flowing water would support,” he says, adding that the dams will worsen the effects of drought.

China, which operates 11 dams along the main stem of the Mekong (or Lancang, as it’s known in China), has come under particular criticism for how it operates its dams in secrecy without much regard for water flow downstream. It is not a member of the intergovernmental Mekong River Commission, which was set up in 1995 to facilitate regional dialogue in the lower Mekong River basin.

China’s decision to halve the water released from its Jinghong Dam for two weeks in July, due to “grid maintenance,” is believed to have contributed in large part to this year’s historically low water levels in the Mekong River. Chinese promises to release more dam water in the future have only served to raise worries over the extent to which China controls the river flow in the Mekong.

“This highlights underlying inequities among Mekong basin countries,” says Sarah Null, a professor at Utah State University in the Department of Watershed Sciences. “Richer nations reap more benefits of hydropower dams, including economic benefits and increased energy supply, while poorer nations are more affected by environmental degradation and reduced food security.”

'Battery of Asia'

Many experts are particularly concerned about the environmental impact of Laos’ plans to turn itself into “the battery of Southeast Asia” by building dozens of hydroelectric dams on the Mekong and its tributaries and selling power to neighboring countries.

Earlier this month, at the same time China reduced the water output from the Jinghong Dam, Laos conducted trials on the giant Xayaburi dam in the northern part of the country, its first hydropower project on the main stem of the Mekong, scheduled to go online in October this year. The trials may have further disrupted the Mekong River’s flow.

One of the poorest countries in the region, Laos already has close to 50 hydroplants operating on various Mekong tributaries and more than 50 planned or under construction, several of them along the main stem of the Mekong. Last year a dam collapsed in southern Laos, flooding large areas and killing dozens of people. Environmentalists have long warned that the Lao projects carry environmental costs that are not fully appreciated or factored in to the decision-making.

“There is a system of total anarchy for hydropolitics and hydropower in the region,” says Eyler, who is the author of the book Last Days of the Mighty Mekong. “There was no overall vision for what the ‘battery of asia’ would look like, and now there is no vision for how that battery will operate.”

Still, there are signs that some countries in the Mekong basin are moving toward alternative forms of energy. Officials in Cambodia have expressed doubts about its plans for two, Chinese-constructed dams on the Mekong River in the northern part of the country, as Cambodia aims to instead increase its solar energy production.

Hogan says the Mekong must avoid the fate of other heavily dammed rivers, like the Colorado in the U.S., which has seen a complete alteration of its natural hydrography and the near total failure of spawning and recruitment of most native fish.

He points out that while the Mekong basin has proven remarkably resilient for many years, it is now facing unprecedented pressures.