What is happening to the Mekong and the Tonle Sap, and why does it matter for Cambodian food? In 2026, the Great Lake started filling six weeks late and stayed low (Mekong River Commission, provisional). Over the long term, the water flowing in has dropped by almost 30% (Quan et al., 2025). This matters because fish gives Cambodians about 70% of their animal protein (FAO Cambodia, 2026).
What happened
In the rainy season, the Mekong rises so high that it pushes water backwards up the Tonle Sap River. This fills the Great Lake. Scientists call this the reverse flow. In a typical year, the lake grows about 6 times bigger, from about 2,500 to about 15,000 square kilometres (Mekong River Commission, estimate). This yearly rise and fall is the flood pulse, the lake's heartbeat.
In 2026, the heartbeat was late. The reverse flow started on 10 July 2026 (MRC, provisional). In 2025, it started on 29 May. That is 42 days later (our calculation, provisional). By 21 September 2026, 35.67 cubic kilometres of water had flowed in (MRC, provisional, season still running).
The lake also stayed low. In August 2025, it held about 98% of its long-term average water (MRC). In June 2026, it held about 69%, and in August 2026 about 59% (MRC, provisional). So about 4 in every 10 litres of the usual water were missing (our calculation, provisional).
A 2025 study found the reverse flow is shrinking over time. It averaged 41.5 cubic kilometres a year in 1996–2009, and 29.3 in 2010–2024. That is a drop of almost 30% (Quan et al., 2025, research estimate).
Why is the flood pulse getting weaker?
Researchers point to four forces.
- Rain. In 2020, the reverse flow was only 58% of its 2008–2021 average. In 2021, it was 51%. The MRC said low rain was the main driver (MRC, 2022).
- Dams upstream. Dams hold water in the wet season and release it in the dry season. In 2019–2021, China's two largest reservoirs held back 10 to 20 cubic kilometres a year. That equals 30% to 60% of an average reverse flow (MRC, 2022, estimate).
- Sand mining. About 32 million tonnes of sand were taken from Cambodia's rivers in 2011, and about 59 million in 2020. The river brings in only about 6.2 million tonnes a year (Quan et al., 2025, estimates). So in 2020, about 9 to 10 times more went out than came in (our calculation). A deeper riverbed lets water rush past the lake, instead of pushing into it. The study estimates this alone cut the reverse flow by 40% to 50% in 1998–2018 (model estimate).
- Farm water. A 2022 study points to irrigation, meaning river water taken to grow crops: about 2.1 cubic kilometres a year in Cambodia's floodplains (Chua et al., 2022, estimate for 1976–2019).
Who is involved
Six countries share the river: China, Myanmar, Laos, Thailand, Cambodia and Vietnam. Four are members of the Mekong River Commission: Cambodia, Laos, Thailand and Vietnam. The MRC collects data, but it cannot order countries what to build. Since October 2020, China shares river and rain data all year from 2 stations (MRC).
The basin has 1,055 dams, and 661 are operating. Together, they can produce up to 36,058 megawatts (Ang et al., 2024, data to 2023). Dams bring electricity and more dry-season water. They also trap sediment, the mud and sand a river carries, and block fish on their journeys (MRC; WWF et al., 2024).
Risks and open questions
- Fish. The Lower Mekong catches about 2.3 million tonnes of wild fish a year, worth about US$11 billion (MRC, estimate). The MRC says the catch is lower in recent years, mostly from the Tonle Sap (MRC, 2023). About 1 in 5 of the Mekong's 1,148 fish species is already threatened, meaning at risk of dying out (WWF et al., 2024, assessment).
- Sediment. Sediment feeds farmland. At monitored Lower Mekong stations, it fell by about half from 2012 to 2022 (MRC, 2023). In an MRC scenario for 2040, with no steps to limit the damage, sediment reaching the Mekong Delta could fall 97% compared with 2007 (forecast).
- The flow. If the riverbed keeps sinking, the reverse flow could fall about 69% by 2038, compared with 1998 (Quan et al., 2025, forecast).
What it means for you
About 70% of the animal protein Cambodians eat comes from fish (FAO Cambodia, 2026, estimate). About 4 in 10 Cambodians rely on the Great Lake and its floodplains for food and income (MRC, estimate).
In 2025, Cambodia produced 978,560 tonnes of fish, up 5.6% from 2024 (FAO Cambodia, 2026, official figure). About half (53%) came from rivers, lakes and floodplains. About a third (33%) came from fish farms, and the rest (14%) from the sea (shares: our calculation).
So the national total is still growing. But the wild fish that many families catch themselves depend on the flood pulse. When the lake shrinks, fishing areas shrink too. That can mean less food for families who fish to eat.
Different views
- The riverbed. Quan et al. (2025) link only about 8% of the reverse-flow decline to changes in upstream flow. They say the deeper riverbed is the main driver (model estimate).
- Upstream changes and farm water. Chua et al. (2022) link only 5% to 21% of the drop in river flow at Kampong Cham and Kratie to upstream changes, meaning dams and rain together. They also point to irrigation (estimate).
- Dams. The Mekong Dam Monitor (Stimson Center), an independent tracker, said that in early September 2026 the river was "about one meter below normal", "mostly due to upstream dam restrictions" (the monitor's assessment).
- Rain. The MRC said low rain was the main driver of the 2019–2021 low flows (MRC, 2022).
- Climate. Supporters of dams argue climate change is the main driver, as reported by the International Crisis Group (2024).
About the data
- Water data runs to 21 September 2026. All 2026 MRC water figures are provisional: the 10 July start, the 35.67 cubic kilometres, the 69% and the 59%.
- Lake volume is a share of the MRC long-term average, not a raw volume.
- The 2026 season total, the long-term yearly average and the 2020–2021 shares are different measures. They cannot be compared directly.
- The figures on causes are research estimates. The 2038 and 2040 figures are forecasts. The 42 days, "4 in 10 litres", "9 to 10 times" and the 53/33/14% shares are our calculations.
- Fish production uses FAO Cambodia's 2025 figures only. Dam data runs to 2023.
