Sand Mining in the Mekong: Raising Cambodian Decision-Makers’ Awareness of the Urgency for Tonlé Sap

The Tonlé Sap, located in Cambodia, is Southeast Asia’s largest freshwater lake and a UNESCO-recognized biosphere reserve. Its water level fluctuates dramatically with the seasons: it expands during the rainy season, sustaining extraordinary biodiversity, then contracts in the dry season, providing vital resources to millions of people.

This lake occupies a crucial place within the Mekong Basin, one of the richest ecosystems on Earth after the Amazon

But this life-giving rhythm—often called the “heartbeat of the Mekong”—is weakening dramatically under the assault of massive sand extraction in the lower basin, in Cambodia and Vietnam.

A study published in Nature Sustainability in November 2025 reveals that this practice reduced seasonal reverse flows into the lake by 40 to 50% between 1998 and 2018—far more than the impact of upstream dams or climate variability.

The full report is available here: https://www.nature.com/articles/s41893-025-01677-8.

The threatened natural miracle of Tonlé Sap

At the heart of this ecological drama lies Tonlé Sap, a unique hydrological jewel. Each monsoon season, from June to October, the swollen Mekong reverses its flow through the Tonlé Sap River, causing the lake to swell to up to five times its normal size—from 2,500 km² to more than 12,000 km².

These floods fertilize flooded forests and wetlands, creating ideal spawning grounds for more than 300 fish species—the backbone of food security and livelihoods for millions of Cambodians.

During the dry season, the lake drains its waters toward the Mekong Delta in Vietnam, delivering freshwater and nutrient-rich sediments that support 23 million people, their rice fields, and aquaculture. This cycle is essential to regional food security, yet it is now faltering.

Sand extraction: the real culprit

Long blamed, Chinese and Lao hydropower dams or climate-driven droughts are in fact only secondary accomplices.

“Upstream dams have a measurable effect, but the primary driver of the decline in Tonlé Sap’s flood pulse is intensive downstream sand extraction,” says Quan Le, a flood-risk researcher at Loughborough University (UK) and lead author of the study.

Using sonar images of the riverbed along with historical water-level and discharge data, the team modeled the entire lower Mekong. The verdict: over two decades, the riverbed has sunk by an average of 2 to 3 meters (6–10 feet) in Cambodia and Vietnam, preventing the river from rising high enough during floods.

In 2020, Cambodia extracted 59 million tonnes of sand—for construction, glass, and electronics—exceeding the river’s natural replenishment capacity, already reduced by dams that trap sediments. “For the same observed discharge, water levels have visibly dropped, a clear sign of bed lowering caused by sand mining,” notes Quan Le.

Cascading consequences: from the lake to the delta

Less reverse flow into Tonlé Sap means an excess of 26 km³ of water rushing toward the delta during the monsoon, intensifying floods and riverbank erosion in one of the most densely populated regions of the Greater Mekong.

In the dry season, outflows from the lake fall by up to 59% below historical averages, worsening saltwater intrusion into low-lying lands and depriving soils of vital nutrients for agriculture. Lake habitats are already shrinking, threatening biodiversity and local fisheries. In the delta, entire communities are losing farms, homes, and livelihoods to erosion and salinization.

Apocalyptic projections by 2038

If extraction continues at current rates, models predict a 69% reduction in flood flows and a 40% shrinkage of the lake’s summer extent compared to 1998.

“We are already seeing the flooded area decline, depriving critical habitats. If the reverse flow collapses entirely, the future of the lake is grim,” warns Steve Darby, professor of physical geography at the University of Southampton and co-author of the study.

Edward Park of Nanyang Technological University (Singapore) hails these “highly significant findings,” the first direct quantification of impacts, and calls for sustainable extraction thresholds.

Insatiable demand fuels the crisis

Asia is consuming sand at a breakneck pace: Singapore imported US$756 million worth of Vietnamese sand between 2009 and 2016.

In Cambodia, official bans conceal an explosive domestic demand—urban land reclamation in Phnom Penh, elevated highways—and persistent illegal mining. Hotspots include downstream of Kratie (Cambodia), the Cambodian-Vietnamese border, around Phnom Penh, and throughout the Vietnamese delta.

Toward life-saving governance?

Quan Le’s team is developing an AI- and satellite-based monitoring system to track dredging vessels and help enforce permits. In partnership with WWF, they are mapping high-risk zones to enable targeted, resilient extraction. “The only long-term solution is to halt and reverse riverbed lowering by restoring sediment balance—ending river mining, implementing controlled dam flushing, and adopting sustainable alternatives such as recycling,” insists Quan Le.

The devastating impacts in the delta may finally raise awareness among Cambodian decision-makers of the urgency facing Tonlé Sap. Time is running out to save the beating heart of the Mekong.