Mekong upstream dams and the decline of Tonle Sap flow reversal
- My flood season became 26–40 days shorter.
- Four dams hold back 81% of my sediment. BusinessDay (via Reuters)
- In my delta, sand has been taken at 6–15 times the rate at which I replace it. Nanyang Technological University (NIE)
- Researchers Stimson Center (US-funded study): Concluded that Chinese dams held back water in 2019. Stimson Center (Eyes on Earth)
- Government and regulators China: Rejected the conclusion.
- International bodies Mekong River Commission: Said the study used calculated flow and the record low came from rainfall; accepted that the dams raised dry-season flows and lowered wet-season flows.
- Researchers Chua et al. (2022): Dams explain about 21% of the 56.5% decline; irrigation and riverbed incision are the other important drivers; no significant rainfall trend over 1960–2019.
- Researchers Earth Observatory of Singapore and NTU: In the Vietnamese delta, sand mining accounts for 25–30% of riverbed erosion and 16–30% of the rise in salinity. Earth Observatory of Singapore
- Government and regulators Ung Dipola, Cambodia's Director General of Mines: Said the erosion does not come from sand and that dredging has taken about 4% of the river. Mongabay
- Press and investigations Mongabay: Reported 49 licences on the Mekong and Bassac rivers and extraction in 2020 at 16 times natural replenishment. Mongabay
- Researchers Singapore Earth Observatory and NTU compilation: Reported about 20 million tonnes a year permitted against more than 50 million tonnes a year extracted on the main stem of the Mekong, about 60% of it illegal. Earth Observatory of Singapore
Each monsoon the Mekong pushes the Tonle Sap River backwards into Cambodia's Tonle Sap lake. A 2022 peer-reviewed study found this reverse flow fell by 56.5% between 1962–72 and 2010–18, and attributed about 21% of the fall to dams.
Chua et al. 2022 (Hydrology and Earth System Sciences). The study attributed about 21% of the decline to dams.
Status As of July 2026: studies attribute the decline to dams, irrigation abstraction and sand-driven riverbed incision; the attribution to Chinese dams was disputed by China in 2019–2020 and no ruling or settlement was found.
- Operators of upper-basin (Lancang) hydropower in Yunnan, China — Operate 11 dams with more than 47 billion m³ of storage.
- Tonle Sap fishers and lower-basin communities — Fish provide more than 60–70% of animal protein in Cambodia; annual catch has been put at 150,000–550,000 tonnes.
- Residents of the Vietnamese Mekong Delta — 1,808 houses lost to the river or sea in 2018–2020; more than 600 hectares of coastal land lost each year. Eco-Business (Mekong Eye)
What happened
The Tonle Sap in Cambodia is a lake whose flow reverses each year. During the monsoon the Mekong pushes back the Tonle Sap River and the lake grows to more than three times its dry-season size, above 15,000 km². The Mekong River Commission has put the lake's annual fish catch at 150,000–550,000 tonnes, and fish provide more than 60–70% of animal protein in Cambodia. For the lower Mekong countries, the Commission's 2007 paper put the share of animal protein from fish at 47–80%.
A study by Chua and colleagues (2022, Hydrology and Earth System Sciences) found that reverse flow into the Tonle Sap fell by 56.5% between 1962–72 and 2010–18, and that the flood season shortened by 26–40 days. It attributed about 21% of the decline to dams. It named irrigation abstraction and riverbed incision as other important drivers, and found no significant trend in rainfall over 1960–2019.
In the upper basin, the Lancang (the upper Mekong in China) carries 11 dams with a combined storage of more than 47 billion m³. After the low flows of 2019, a US-funded study published through the Stimson Center in April 2020 said Chinese dams had held back water. China rejected this. The Mekong River Commission said the study used calculated rather than measured flow and that the record low came from low rainfall, but it accepted that the dams raised dry-season flows and lowered wet-season flows.
Sand extraction is the other driver the sources describe. In the Vietnamese Mekong Delta, a study published in Science Advances in July 2026 estimated average extraction of 48 million m³ a year in 2017–2021, 6–15 times natural replenishment, and attributed 25–30% of riverbed erosion and 16–30% of the rise in salinity to sand mining. In Cambodia, satellite-based work reported extraction of the Mekong rising from 24 to 59 million tonnes a year between 2016 and 2020.
A Reuters and EOMAP analysis reported in 2022 that four dams retain 81% of the Mekong's sediment. A Mekong River Commission scenario for 2040 projects a 97% fall in sediment reaching the delta and a loss of between 40% and 80% of fish biomass.
- 1962Start of the 1962–72 baseline period used to measure Tonle Sap reverse flow
- 2007Mekong River Commission paper: fish provide 47–80% of animal protein in the lower Mekong countries
- 2016Cambodian Mekong sand extraction measured at 24 million tonnes a year (later satellite work)
- 2017Cambodia bans sand exports; the ban is lifted in 2020
- 2019Low flows in the Mekong; dispute over the role of upstream dams begins
- 2020Stimson Center publishes the study saying Chinese dams held back water (April); Cambodian extraction measured at 59 million tonnes a year
- 2022Chua et al. report a 56.5% fall in Tonle Sap reverse flow; Reuters and EOMAP report that four dams retain 81% of sediment
- 2026Science Advances study quantifies sand mining in the Vietnamese delta (July)
What was said
Where accounts differ
Each party's account, side by side, with its source. The record does not settle these points; it shows them.
Concluded that Chinese dams held back water in 2019.
Stimson Center (Eyes on Earth)Rejected the conclusion.
Said the study used calculated flow and the record low came from rainfall; accepted that the dams raised dry-season flows and lowered wet-season flows.
Dams explain about 21% of the 56.5% decline; irrigation and riverbed incision are the other important drivers; no significant rainfall trend over 1960–2019.
In the Vietnamese delta, sand mining accounts for 25–30% of riverbed erosion and 16–30% of the rise in salinity.
Earth Observatory of SingaporeSaid the erosion does not come from sand and that dredging has taken about 4% of the river.
MongabayReported 49 licences on the Mekong and Bassac rivers and extraction in 2020 at 16 times natural replenishment.
MongabayReported about 20 million tonnes a year permitted against more than 50 million tonnes a year extracted on the main stem of the Mekong, about 60% of it illegal.
Earth Observatory of SingaporeHow it looked
Shown
Recorded
- Tonle Sap reverse flow fell by 56.5% between 1962–72 and 2010–18, and the flood season shortened by 26–40 days.
- The Chua et al. study attributed about 21% of the decline to dams, named irrigation and riverbed incision as other drivers, and found no significant rainfall trend over 1960–2019.
- Four dams retain 81% of the Mekong's sediment (Reuters and EOMAP, 2022).
- Sand extraction in the Vietnamese delta averaged 48 million m³ a year in 2017–2021, 6–15 times natural replenishment.
- In the Vietnamese delta, 1,808 houses were lost to the river or sea in 2018–2020, and more than 600 hectares of coastal land are lost each year.
- Cambodia's Mekong and Bassac rivers carried 49 sand licences; extraction in 2020 reached 16 times natural replenishment.
Who gained
Reached nature or the ground
Gained elsewhere
Responses on record
- China (government) (2020): Rejected the 2020 study's conclusion that its upstream dams held back water in the 2019 low-flow period.
- Mekong River Commission (2020): Said the 2020 study used calculated flow and that the record low came from low rainfall, while accepting that the dams raised dry-season flows and lowered wet-season flows.
- Cambodia, Director General of Mines (Ung Dipola) (2022): Said riverbank erosion does not come from sand and that sand dredging has taken only about 4% of the river. · Mongabay
In court and with insurers
Lawsuits and rulings
Insurers
What nature saw
Nature's report opens this page. Below: further outcomes measured on the ground.
What we don't know
Nature
Court
- None on record No court case on record.
Figures
- Insufficient Reverse flow decline bed lowering pct: Riverbed lowering 1998–2018, reported from a paper that could not be checked.
Record
- None on record Share of the Tonle Sap decline caused by each driver: dams account for about 21% in the Chua et al. study; the remainder is attributed to irrigation and riverbed incision without a published breakdown in the sources reviewed.
- None on record Who holds the sand licences and extracts sand on the Tonle Sap and Cambodian Mekong stretches: the owners are not named in the record for the Tonle Sap decline.
- Insufficient A 2025 Nature Sustainability paper (Southampton) is reported to say that riverbed lowering in 1998–2018 cut reverse flow into the Tonle Sap by 40–50% and that this would fall further by 2038. The paper could not be checked and the figures are reported but not independently verified.
- Insufficient A figure of about 6.18 million tonnes of sand entering the delta each year against about 50 million tonnes extracted is reported from a summary of a 2020 Nature Sustainability paper; it is reported but not independently verified.
- Unclear Whether the Mekong River Commission's 2040 sediment and fish-biomass scenario figures (97% and 40–80%) have been independently checked is not stated in the sources reviewed.
Did the harm stop?
The sources reviewed record no measure that has stopped the decline; the cited studies describe the driver trends as continuing.
Who paid?
The sources reviewed point to lower-basin fishing communities and delta residents, including 1,808 houses lost in the Vietnamese delta in 2018–2020.
Sources
Press and investigations
- Chinese dams ruin Mekong river delta farmers (Reuters, with EOMAP analysis)BusinessDay (via Reuters) · 2022-12-15
- Mining the Mekong: land and livelihoods lost to Cambodia's thirst for sandMongabay · 2022-08-29
- In Vietnam's Mekong Delta, sand mining means lost homes and fortunesEco-Business (Mekong Eye) · 2022-07-06
- Mekong Delta loses 600 ha of land every year due to landslideRadio Free Asia (Vietnamese service) · 2024-08-24
Communities and civil society
- New evidence: how China turned off the Mekong tapStimson Center (Eyes on Earth) · 2020-04
Researchers
- Sand mining is destabilising the Mekong Delta far more than previously understoodNanyang Technological University (NIE) · 2026-07
- Counterfactual modeling isolates sand mining impacts, revealing it as a key driver of Mekong Delta destabilizationEarth Observatory of Singapore ·
- River sand mining outstrips nature's supply: studyEarth Observatory of Singapore · 2026-07-22
International bodies
- Mekong River Commission technical paper on fisheries and food (2007)Mekong River Commission · 2007
Connected cases
- Cambodian sand exports to Singapore (Koh Kong, Mekong, Bassac) Cambodia · 2007–2026 Related matter
- Four Mekong dams trap an average 81% of sediment, Reuters reports Mekong basin · date not stated Related matter
- Mekong sand mining (Cambodia, Vietnam) Cambodia and Vietnam · 2007–2026 Related matter
- China is building a mega dam in Tibet, upstream of India and Bangladesh China · 2024
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: Building the dam, per Radio Free Asia
- China's natural forest logging ban and the shift of timber imports China · 1998–2014
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: introduced the 1998 logging restrictions and the natural-forest logging ban
- Fengyun-1C anti-satellite test China · 2007–2026
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: State that conducted the test
- Fukushima treated water discharge and Chinese seafood ban Japan · 2023–2025
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: Banned Japanese seafood imports
- Human Rights Watch reports about 930,000 relocated in Tibet for ecological protection China · 2024
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: authority that carried out the relocations, as reported
- Myanmar Kachin heavy rare earth mining and Kok River arsenic Thailand · 2025
- Mekong River Commission (International organisation) — here: Intergovernmental river basin body; source of fish and scenario figures; there: International organisation; assessed arsenic contamination
- Non-peer-reviewed study links China's 1958 sparrow campaign to famine deaths China · 1958–1960
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: ran the Four Pests campaign
- World Bank-backed dams displaced people, and reviews found incomes often not restored Global · 1985–2018
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: Host government for the Three Gorges dam
- Yangtze ten-year fishing ban China · 2020–2030
- Government of China (Government) — here: Operator of the upper Mekong (Lancang) dams; rejected the 2019 low-flow attribution; there: Regulator; ban and compensation