How dams built by China starve the Mekong River Delta of vital sediment

The analysis reinforces an estimate by the Mekong River Commission, set up in 1995 by countries bordering the river, that in 2020 only about a third of those river-borne soils would reach the Vietnamese floodplains. At the current rate of decline, the commission estimated, less than five million tonnes of sediment will reach the delta each year by 2040.

Stretching nearly 5,000 kilometers from the Plateau of Tibet to the South China Sea, the Mekong is a farming and fishing lifeline for tens of millions as it swirls through China, Laos, Myanmar, Thailand and Cambodia before reaching Vietnam.

“The river is not bringing sediment, the soil is salinized,” said Cung, 60, who has grown rice at his family’s 10-hectare farm for more than 40 years.

“Without sediment, we are done,” he said. His diminishing harvest now brings in barely half of the 250 million dong ($10,636) annually that he earned just a few years ago, and his two children and several neighbors have left the area to seek more stable and lucrative work elsewhere.

For decades, scientists and environmentalists have warned upstream dam projects jeopardise livelihoods in a region of some 18 million people and an annual rice market of $10.5 billion that is a major food source for up to 200 million people across Asia, according to WWF estimates, Reuters calculations and Vietnam’s Chamber of Commerce and Industry.

Worry shared by Lower Mekong nations has already led Cambodia to pause plans for two dams on the river, according to the Mekong Dam Monitor, an online platform that provides real-time data on dams and their environmental impact.

But in China and Laos, dam-building goes on. Of seven new dams planned in Laos, at least four are co-financed by Chinese companies, according to Mekong Dam Monitor data.

China’s foreign ministry said the country accounted for only a fifth of the total Mekong basin area and only 13.5% of the water flowing out of the Mekong’s estuary, adding that there was already a “scientific consensus” on the impact of China’s upstream dams. The ministry did not address the slide in sediment levels or the role of Chinese dams in that decline.

Using data derived from thousands of satellite images, EOMAP and Reuters analyzed sediment levels around four major dams on the Mekong, two in China and two in Laos. The analysis showed each dam drastically reduced the sediment that should have otherwise flowed through at those locations – by an average of 81% of the sediment load across the four dams.

“The dams are trapping sediment … each one traps a certain amount, so there isn’t enough reaching the floodplains,” said Marc Goichot, a WWF river specialist in Vietnam who was not involved in the analysis but reviewed the results.

“Sediment and deltas should be able to regenerate and rebuild themselves,” he said. “But the pace at which the natural balance is being forced to change in the Mekong is too fast for the sediment to keep up.”

‘Wake-up call’

Farmers in the Vietnamese Mekong River Delta region were not prepared for the speed at which their landscape – and fortunes – have changed.

The area under rice farming has shrunk by 5% in the last five years alone, with many forced to adopt shrimp farming in salty seawater as an alternative.

Incomes in this once-booming region are now among Vietnam’s lowest, even as the national economy grows at a projected 8% for 2022. The region has seen more outward migration than any other in Vietnam since 2009, according to Vietnam’s Chamber of Commerce and Industry.

The Mekong River Commission estimated in 2018 that total sediment flow by now would be around 47 million tonnes per year. It could be far lower – estimated at just 32 million tonnes per year, according to scientific studies from the last decade including one published in July 2021 in the journal Nature Communications.

“In the past three or four years there has been a wake-up call about sediment,” said the head of the commission, Anoulak Kittikhoun of Laos. “We definitely cannot return to sediment levels seen in the past. We need to preserve what we have.”