- Surging demand for sand used in construction projects poses an existential threat to Southeast Asia’s largest freshwater lake, new research indicates.
- The seasonal expansion and contraction of Cambodia’s Tonle Sap Lake is often referred to as the Mekong River’s “heartbeat” due to its fundamental role in sustaining ecosystems and human lives across the region.
- Sand mining in the Mekong River, particularly in Cambodia and Vietnam, has deepened the river channel, effectively halving wet season flows into Tonle Sap Lake between 1998 and 2018, the study found.
- The stark findings underscore the severity of sand mining impacts, adding urgency to calls for improved and coordinated river governance throughout the Mekong Basin.
Rampant sand mining in the Mekong River is directly weakening critical seasonal river flows that sustain Southeast Asia’s largest freshwater lake, new research indicates.
The Mekong’s annual wet season flood pulse that feeds water into Cambodia’s Tonle Sap Lake has been dwindling year by year.
Experts have long pointed to upstream hydropower dams in China and Laos that trap sediments and alter the Mekong’s flow, combined with droughts intensified by climate change, as major drivers of the gargantuan river system’s declining vitality.
A new study by researchers from the U.K. and Vietnam now shows that sand mining in the Lower Mekong Basin countries of Cambodia and Vietnam has a far greater impact on the flood pulse-lake dynamics than previously understood.
“Upstream dams do have a measurable effect,” said lead author Quan Le, a flood risk researcher at Loughborough University in the U.K. “However, the primary driver of the declining Tonle Sap flood pulse is extensive downstream sand mining.”
The Mekong’s heartbeat
Tonle Sap Lake, a UNESCO Biosphere Reserve, lies within the lower basin of the Mekong River, the world’s second-most biodiverse aquatic ecosystem (after the Amazon). Each wet season, the lake swells up to five times in size as the Mekong’s annual flood pulse surges up the Tonle Sap River, reversing its flow. The situation then flips during the dry season, when water flows out of the lake downstream into the densely populated Mekong Delta.
This rhythmic expansion and contraction is often referred to as the Mekong’s “heartbeat” due to its fundamental role in sustaining ecosystems and human lives across the region.
The wet season floodwaters permeate Tonle Sap Lake’s marginal forests and wetlands, creating crucial breeding grounds and nursery habitats for more than 300 species of fish that nourish and sustain the livelihoods of millions of local residents. Conversely, in the dry season, many of the roughly 23 million people who live in the Mekong Delta depend on the downstream flow of freshwater and fertile sediments to replenish riverbanks, rice fields and aquaculture ponds.
A near-halving of wet season flows
Dredging of sand from the Mekong’s riverbed for construction, glass and silicon-based manufacturing has surged in recent years. Some 59 million metric tons of sand were extracted from Cambodia’s stretch of the river in 2020 alone, a separate study revealed in 2023.
The volume of sand extraction now surpasses what the river can naturally deliver, Quan and his colleagues conclude in their study, a situation compounded by a lack of replenishing sediments due to dams. The result: a relentless lowering of the riverbed.
Quan and his colleagues modeled the entire Lower Mekong Basin using sonar-derived imagery of the Mekong riverbed and long-term water level and river discharge data, calculating the river bottom has dropped by an average of 2-3 meters (about 6-10 feet) across Cambodia and Vietnam over the past two decades.
This deepening of the river channel prevents the river from rising as high during the wet season, effectively reducing the Mekong’s wet season reverse flow into Tonle Sap Lake by 40-50% percent annually between 1998 and 2018, the team report in their study.
“For the same observed river flow, water levels have dropped noticeably over time, providing clear evidence of riverbed lowering caused by sand mining,” Quan said.
With less water flowing into the lake system, there’s a surplus 26 cubic kilometers (6.2 cubic miles) of water flowing downstream into the Mekong Delta during the monsoon season, the study says, increasing the risk of flooding and riverbank erosion in one of the most densely populated parts of the Greater Mekong region.
Furthermore, dry season flows out of the lake to the delta are up to 59% lower than historical averages due to the riverbed lowering. This exacerbates already problematic tidal and saltwater intrusion into low-lying farmland, the study says, and reduces the volume of nutrient-laden sediments that would otherwise naturally fertilize agricultural land.
Action needed to avert further decline
Projecting their models into the future, the researchers calculated that if sand mining continues unchecked, by 2038 the flood pulse could decline by up to 69% and the wet season size of the lake will shrink by 40% compared to the 1998 baseline.
Such an eventuality could be disastrous for the lake system, risking its near-total collapse, according to Steve Darby, a professor of physical geography at the University of Southampton in the U.K. and co-author of the new study.
“We are already seeing the area of the lake inundated declining, meaning there is less critical habitat available,” Darby told Mongabay in an email. “If the reverse flow were to decline still further — or even collapse completely — the future of the lake looks very bleak indeed.”
To mitigate the risks, the researchers call for greater efforts to halt and reverse the riverbed lowering. There’s still time to limit the worst consequences, they say, by stepping up sustainable sediment management throughout the Mekong Basin.
“The only long-term solution is to stop and reverse the riverbed lowering that weakens the Tonle Sap’s natural flood pulse,” Quan said. “This means restoring the Mekong’s sediment balance by reducing or ending river sand mining and increasing the amount of sediment released from dams through controlled flushing.” In addition, he said, the construction industry should prioritize the development of “smarter, sustainable alternatives” to sand and recycling more materials.
Sand demand surges
Asia’s demand for sand shows no signs of abating, however, with pressure coming from both within and outside of the Mekong region. Singapore reportedly imported $756 million worth of “stone, sand, and gravel” from Vietnam between 2009 and 2016, for example, highlighting gaps in the enforcement of sand export bans introduced in Vietnam in 2009.
Cambodia has also banned the export of sand, at least on paper. Yet such policies don’t address persistent domestic demand for sand that seems set to expand. “Domestic demand continues to grow,” Quan said, noting massive urban land reclamation, as seen in Phnom Penh and highway projects to raise roads above the Vietnam Delta plain. “Illegal sand mining also persists despite [the presence of] regulations, further undermining efforts to halt riverbed lowering.”
Quan said it was tricky to pinpoint the most egregious hotspots of sand mining, given the mobility of dredging vessels over vast distances. However, he noted intense activity downstream of Kratie in Cambodia that has likely contributed to lower water levels at Phnom Penh, a crucial hydrological location that determines the reverse flow into Tonle Sap Lake.
Edward Park, an environmental scientist at Nanyang Technological University in Singapore, who wasn’t involved in the study, described the new findings as “very meaningful.” In a 2021 study, Park had inferred that sand extraction in the lower basin disrupted reverse flows into Tonle Sap Lake, cautioning against sacrificing the Mekong system’s social and biological health for short-term economic gains from sand mining. He noted the new study provides the first direct quantification of these impacts.
“The new study shows that sand mining around Phnom Penh is problematic for the Tonle Sap reverse flow,” Park said. “Therefore, the government should set thresholds based on sustainable extraction rates that can be calculated and should then monitor and enforce those restrictions.”
Toward better governance
Quan’s team is now developing a vessel-tracking system that uses machine learning and satellite images to identify and track sand mining vessels across the Lower Mekong Basin. Early analyses using these methods indicate hotspots of sand mining at the Cambodia-Vietnam border, around Phnom Penh, and in various locations in the Vietnam Delta.
“We hope that this type of system could be used to help agencies monitor compliance [with sand mining permits] in a step towards better governance of sand extraction in the Mekong system,” Darby said.
The team has also been working with WWF to provide planning authorities in the delta with maps showing where sand mining is most likely to trigger severe impacts, so that extraction can be focused in more resilient areas, informing a more risk-based approach to sand extraction.
The scale of the impacts unfolding in the Vietnam Delta have likely sharpened the focus of policymakers on finding solutions, Darby said, noting how riverside communities have lost farmland, homes and other properties to severe erosion and saltwater intrusion.
“I think the scale of these impacts have focused minds,” he said. “Perhaps the scale of impacts associated with the threat to the Tonle Sap lake may also impress the urgency of the problem on to the government of Cambodia.”
Banner image: The University of Southampton research vessel (right) passing sand barges on the Mekong River near Phnom Penh, Cambodia. Image courtesy of Andy Ball / University of Southampton.
Carolyn Cowan is a staff writer for Mongabay.
Citations:
Quan, L. Q., Hackney, C. R., Vasilopoulos, G., Coulthard, T., Hung, N. N., Darby, S. E., & Parsons, D. R. (2025). Sand-mining-driven reduction in Tonle Sap Lake’s critical flood pulse. Nature Sustainability, 8(12), 1455-1466. doi:10.1038/s41893-025-01677-8
Hackney, C. R., Vasilopoulos, G., Heng, S., Darbari, V., Walker, S., & Parsons, D. R. (2021). Sand mining far outpaces natural supply in a large alluvial river. Earth Surface Dynamics, 9(5), 1323-1334. doi:10.5194/esurf-9-1323-2021
Beckwith, L., Marschke, M., Darbari, V., & Hackney, C. R. (2025). Exploring how sand infill is transforming Phnom Penh’s Tompun-Cheung Ek wetland. The Extractive Industries and Society, 24, 101713. doi:10.1016/j.exis.2025.101713
NG, W. X., & Park, E. (2021). Shrinking Tonlé Sap and the recent intensification of sand mining in the Cambodian Mekong River. Science of The Total Environment, 777, 146180. doi:10.1016/j.scitotenv.2021.146180
See related story:
Mining the Mekong: Land and livelihoods lost to Cambodia’s thirst for sand
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