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Upstream Chinese reservoirs full but lower Mekong in drought

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Upstream Chinese reservoirs full but lower Mekong in drought

In Thailand, farmers complain that their crops won’t grow properly. In Cambodia, fishers say their catch is smaller than ever. And in Laos, unexpected water fluctuations have played havoc on livelihoods.

But while low river levels have plagued the lower Mekong River through August and most of September, China faced no such shortage. In the first week of September, two massive dams that sit on the upper Mekong, or Lancang, as it is known in China, restricted more than 5 billion cubic meters of water.

That represented the most water ever held back in a single week, according to the Washington-based Stimson Center, whose Mekong Dam Monitor records river levels, reservoir volume and precipitation across the river basin.

Releases from smaller Chinese dams that began last week have helped Mekong levels rise on the Lao-Thai border by two meters, while increasing rains downstream also helped low levels recover somewhat.

But for much of August and September, parts of the lower Mekong were reaching record lows as China’s hydropower reservoirs continued to grow. Between July 18 and September 3, China’s Xiaowan Dam reservoir increased in size from 6.7 billion cubic meters to approximately 14.3 billion cubic meters of water, according to the Mekong Dam Monitor.

The water being held back exacerbated a downstream wet season drought, said Brian Eyler, Southeast Asia program director at Stimson.

Forecasts predict that El Niño will bring a dry fall, so it’s likely that China’s dam operators took the water when it was available in August,” said Eyler. “Last year [due to drought] they weren’t able to fill up to adequate levels, I imagine there is pressure on dam operators to fill up as much as they can…. The tragic issue is that it’s needed desperately downstream.”

Much of the lower Mekong has been suffering from diminished water flow, but no areas have been as hard-hit as the communities along the Lao-Thai border.

Gauges at Chiang Sen in Thailand’s Chiang Rai province showed the river’s level about 3 meters below the average the first two weeks of September.

Instead of water coming in as part of the natural river flow, the restrictions in the large reservoirs upstream meant levels were rising through the summer only when there was rain or flash floods. But because this rainy season had been so dry, levels remained extremely low.

“It’s not normal,” said a villager from Xayaburi province. “It depends on dams opening and closing the water gates, and this year there is not much rain.”

Sothea Khem, a river flood forecasting specialist at the Mekong River Commission, an inter-government agency that works with regional governments to manage river resources, noted that until rains picked up last week, most stations in the region showed water levels well below the long-term average.

He said climate change was having a far larger effect on downstream levels, noting that the total inflow from China is typically 16% to 25%.

“The lower Mekong basin is much more influenced by rainfall. If you think about the inflow from China, of course, it impacts. Nevertheless, we must consider how our tributaries’ inflow contributes to our main Mekong basin region.”

While inflows from China may account for a smaller fraction of the basinwide levels, China’s contribution at Chiang Saen is nearly 100 %, pointed out Eyler.

“Climate impacts are typically the most significant, and dam restrictions work together with a lack of rain during the dry season to drive low river levels. However we found that in August 2023 dam restrictions were far greater than lack of rain on driving low levels,” he wrote in an email.

It is precisely that combination of upstream water restrictions and regional changes to the climate that has environmentalists concerned over the long-term impacts on the region’s biodiversity.

Located at the northern edge of Cambodia’s Stung Treng province at the border of Laos sits a 14,600-hectare Ramsar, or protected wetland, site. It boasts flooded forests, deep Mekong pools and sandy islands. But the delicately balanced ecosystem has been badly unsettled by the Mekong River damming.

“In the past, by June and July the river was full of water, but now it is already September, the water is still at the bottom of the river,” environmentalist Ly Vichetra told RFA. “When the water is high, the fish spawn more and more. As the water level changes the fish perish accordingly.”


China has for years promised more transparency around how it stores and releases water. On Sept. 10, China and five other Mekong countries signed an agreement to share water flow data from their dams.

In an email to RFA, an MRC spokesperson explained a key purpose was “to be able to prepare for climate risks and the need to operate the reservoirs accordingly in order to minimize the risks to the downstream communities.”

But such data has been shared for years, and it has not yet resulted in improved conditions downstream, said a Lao environmentalist who spoke on the condition of anonymity for security reasons.

“The important data is about the data on water release from dams,” he said. “Another one is data on fish migration, environment [impacts] and fisheries.”

Citizens in downstream countries said they struggle with rapidly changing water releases that come with little or no warning and have hurt their livelihoods. These include not only China’s dams, but dams throughout the lower Mekong and on its tributaries.

A villager in Laos’ Xayaburi province said that unexpected water releases have damaged boats and washed away fishing equipment in his community.

“If we know when the dams will release water, we will have time to prepare. One or two days in advance, the people will feel calm.”

“One thing that we want to know the most is data on water release,” echoed a villager in Thailand’s Nong Khai province. “These days, the water level along the Mekong river is always up and down. Even if it is raining heavily, once dams in China keep water, the water level in the Mekong is suddenly down.”

Leang Bunleap, executive director of the 3S Rivers Protection Network, a local NGO that has studied the impacts of dams on the Mekong and its tributaries, said the Cambodian government and the MRC should do more to find ways to reduce transboundary environmental impacts. That includes pressing China to take downstream needs into account.

The releases that began last week may well reflect that recognition from China.

“Cambodia should inform the upstream countries to respect the international law, let them keep the flow as natural or prepare to give and share information and cooperate with other countries to make the water management more efficient.”

  • RFA report, Sept 19, 2023