How China-built dams starve the Mekong River Delta of vital sediments

How China-built dams starve the Mekong River Delta of vital sediments

How China-built dams starve the Mekong River Delta of vital sediments

By Kanupriya Kapoor, Simon Scarr and Phuong Nguyen

SOC TRANG, Vietnam (Reuters) – Standing on the bank of the Mekong River, Tran Van Cung can see his rice paddy washed away before his eyes. The edge of the paddy field is crumbling into the delta.

Just 15 years ago, Southeast Asia’s longest river carried about 143 million tons of sediment each year – as heavy as some 430 Empire State Buildings – through the Mekong River Delta, dumping nutrients along the riverbanks essential for keep tens of thousands of farms like Cung’s intact and productive.

But as hydroelectric dams built in China have mushroomed upriver, much of that sediment has been locked up, an analysis of satellite data from German aquatic remote sensing company EOMAP and Reuters shows. (Graphic – Starving the Mekong: )

The analysis bolsters an estimate by the Mekong River Commission, established in 1995 by countries bordering the river, that in 2020 only about a third of that riverine soil will reach Vietnam’s floodplains. At the current rate of decline, the commission estimates, fewer than five million tons of sediment will reach the delta each year by 2040.

Stretching nearly 5,000 kilometers from the Tibetan Plateau to the South China Sea, the Mekong is an agricultural and fishing lifeline for tens of millions of people as it passes through China, Laos, Myanmar, Thailand and Cambodia before reaching Vietnam.

“The river carries no sediment, the soil is salinized,” said Cung, 60, who has been growing rice on his family’s 10-hectare farm for more than 40 years.

“With no sediment, we’re done,” he said. Her dwindling crop now brings in barely half of the 250 million dong ($10,636) a year she earned just a few years ago, and her two sons and several neighbors have left the area to seek more stable work and profitable elsewhere.


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

Concern shared by Lower Mekong nations has already led Cambodia to suspend 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 continues. Of the seven new dams planned in Laos, at least four are co-financed by Chinese companies, according to data from the Mekong Dam Monitor.

China’s foreign ministry said the country accounts for only one-fifth of the total area of ​​the Mekong basin and only 13.5 percent of the water flowing from the Mekong estuary, adding there was already a “scientific consensus” on impact of Chinese upstream dams. The ministry did not address the decline 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 that each dam dramatically reduced sediment that would otherwise have flowed there, by an average of 81 percent of the sediment load across the four dams.

“Dams trap sediment… each one traps a certain amount of it, so there isn’t enough of it to reach the floodplains,” said Marc Goichot, a riverine specialist at WWF Vietnam who was not involved in the analysis. but he revised the results.

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


Farmers in Vietnam’s Mekong River Delta region were unprepared for how quickly their landscape and fortunes have changed.

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

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

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

“There has been a wake-up call about sediments in the last three or four years,” said commission head Anoulak Kittikhoun of Laos. “We definitely can’t go back to sediment levels seen in the past. We have to preserve what we have.”

Meanwhile China, eager to increase renewable energy capacity to reduce its reliance on coal, has already built at least 95 hydroelectric dams on tributaries that flow into the Mekong, called Lancang in China.

A further 11 dams have been built since 1995 on the same main river in China – including five mega-dams each more than 100 meters high – while China has helped build two in Laos.

Dozens more are expected. The state-owned Huaneng Lancang River Hydroelectric Power, charged with developing resources, aims to double the grid’s 21.3 gigawatt capacity by 2025, its chairman Yuan Xianghua told Reuters.


EOMAP and Reuters’ analysis of satellite images taken over three decades around four major dams in China and Laos has found evidence that the dams dramatically reduce sediment flow.

The analysis was based on the turbidity measurements depicted in the images – the amount of light scattered by solid particles suspended in the water – as a proxy for sediment levels. Sediments cloud the water as it flows: the muddy the water, the greater the turbidity and the more likely it is to carry sediment.

EOMAP used the same approach to measure sediments in the Elbe River in 2010 and in hydroelectric reservoirs in Switzerland and Albania in 2021. Its findings on those streams matched ground-based observations.

Satellite images for analysis of the Mekong date back to the 1990s, which “allows us to calculate turbidity levels before many dams are built,” said EOMAP data analyst Philipp Bauer.

After discarding images obscured by cloud cover or pollution, the team were left with 1,500 depicting turbidity around two dams in China and two in Laos. Experts not involved in the analysis agreed that the findings made it clear that dams were a major culprit in the delta’s sediment loss.

“Traditional dams capture everything,” said economist Brian Eyler of the Stimson Center, which runs the Mekong Dam Monitor. “China has 11 in the mainstream, plus other countries, so all of these are working together to reduce the sediment load.”

For example, before China built its fourth largest dam at Nuozhadu in Yunnan province, water turbidity measurement in 2004 averaged 125.61, the so-called “nephelometric units of turbidity”, or NTU , according to satellite data.

After the dam was completed in 2012, the average turbidity at the same spot plummeted by 98% to just 2.38 NTU, clear enough to meet the World Health Organization’s classification for drinking water.

The Xayaburi and Don Sahong dams in Laos are the most recent to be put into operation, with Xayaburi being the largest on the entire Mekong River. The average turbidity before China built the Xayaburi Dam was 101.51 NTU, while after the dam went online in 2019, the turbidity dropped by 95% to an average of 5.16 NTU.

And at Laos’ southern border with Cambodia, turbidity dropped by about 42% to 42.39 NTU after the Don Sahong Dam started up in 2019.

Reuters asked the Chinese and Lao governments about the impact of their dams and plans to build more. China’s foreign ministry did not respond to questions about its existing and planned dams or their impact on sediment levels, while the Lao government did not respond to requests for comment.

The governments of other countries through which the Mekong flows also did not respond to requests for comment.


In Vietnam’s Cung rice paddy, riverbank seedlings have little time to take root before falling into the water as the banks give way.

Located about 430 kilometers (270 miles) from the nearest upstream dam – and about 1,400 kilometers from the Chinese border – the agricultural area’s turbidity has dropped by about 15 percent over the past 20 years, to about 61 NTU on average today, according to analysis by EOMAP and Reuters.

Downstream countries affected by sediment dwindling have lobbied unsuccessfully for China to share data on sediment flows and details of its dam-building plans. Beijing only shares data on water levels and flows from its major dams.

Last year, the Mekong River Commission launched its own joint study with China to examine the impacts of dams, but the results won’t be known until 2024.

But while the commission expressed concern about sediment depletion, “we haven’t had a serious conversation [with China] still on the sediments,” said the head of the Kittikhoun commission.

“Water flow is a priority. Working with China, you have to take it one step at a time.”

Sitting cross-legged by the river, rice farmer Cung said he and his colleagues have struggled to find information on how to adapt to the changes caused by the dams.

“It’s not an easy decision to make, but sometimes quitting is the only sensible economic choice,” Cung said.

($1 = 23,505.0000 dong)

(Additional reporting by David Stanway, Claire Trainor and Manas Sharma; Editing by Katy Daigle and Kenneth Maxwell)

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