Posted on November 12, 2025
Cambodia’s Tonlé Sap supports important fisheries and wildlife
The surging demand for sand threatens to dramatically shrink Cambodia’s Tonlé Sap, Southeast Asia’s largest freshwater lake. Mining that extracts sand for concrete, glass, and landfills are altering the flow of the Mekong River, a new study finds, which could choke off water to one of the region’s most important wildlife habitats and fisheries that support millions of people.
The sand mining problem “has been documented before but is not well understood,” says Brian Eyler, a water policy specialist at the Stimson Center, a think tank in Washington, D.C. He hopes the new study “will elevate this issue to levels where it will get good and useful attention.”
An unusual hydrological phenomenon underlies the biological richness of Tonlé Sap lake. Sitting in central Cambodia northwest of Phnom Penh, it is linked to the Mekong by the 120-kilometer-long Tonlé Sap River. During the dry season, from November to April, the river drains the lake, which shrinks to a shallow body a few thousand square kilometers in size, holding 1 to 2 cubic kilometers of water. But during the wet season from May to October, rising Mekong levels reverse the flow of the Tonlé Sap River, feeding the lake instead of draining it. The flood pulse expands the lake’s surface area by four to six times and swells its water volume to 80 cubic kilometers.
Nutrients in the floodwaters help support fisheries that historically yielded annual catches of up to 250,000 tons, although they have sharply declined in recent years. The lake helps make the Mekong “likely the most diverse system globally for migratory freshwater fish,” says Alice Hughes, a conservation biologist at the University of Melbourne. She notes that the river and lake host about 1400 fish species, one-quarter of them unique. Tonlé Sap wetlands support rare and endangered waterbirds and, during the dry season, water from the lake helps augment the Mekong’s flow, preventing saltwater from creeping up the Mekong delta.
In the early 2000s, researchers noticed the flood pulse was weakening and speculated that climate change and a proliferation of dams were to blame. But those two factors could not explain the magnitude of the decline, environmental scientists Wen Xin Ng and Edward Park of Nanyang Technological University reported in 2021 in Science of the Total Environment. Instead, they blamed the dredges that are scooping and sucking up sand from the bed of the Mekong near Phnom Penh. By deepening the river’s channel, they said, the sand mining prevents the river from rising as high during the wet season, reducing the Tonlé Sap River’s reverse flow.
The new study, published today in Nature Sustainability, takes a closer look at that idea. A team led by river delta hydrologist Quan Le of Loughborough University compiled data on sand dredging, river depths, and water levels. The researchers then used a hydrological model of the entire lower Mekong basin to simulate several scenarios. The modeling results indicate “a clear relationship between sand mining—induced riverbed lowering and the reduction of the Tonlé Sap’s flood pulse,” Le says.
One simulation found that if mining continues at its current pace, by 2038 the flood pulse volume will decline by 69% and the wet season size of Tonlé Sap lake will shrink by 40%. In addition, dry season outflows to the Mekong delta will drop by 59%. “The only effective mitigation for these problems is to halt and reverse the riverbed lowering,” Le and colleagues write.
The study authors conclude that sand mining poses “an existential threat” to Tonlé Sap lake. The problem extends far beyond Cambodia. After water, sand is the world’s second most exploited resource, often extracted from riverbeds or shores. But “the environmental, economic, and social costs of sand mining likely exceed the benefits by an order of magnitude,” says fish biologist Zeb Hogan of the University of Nevada, Reno. Many governments have taken notice, he adds. “In regions where the impacts of sand mining have been well studied, large scale in-river extraction is almost always restricted or prohibited.”
In Cambodia and adjacent Vietnam, officials have already banned the export of sand. But Le says the country continues to fill the rapidly growing domestic demand for sand by dredging—and harming—the Mekong.
