Intense sand mining is putting the largest freshwater lake in Southeast Asia at risk of collapse with catastrophic consequences, a new study has found.
The huge Tonlé Sap Lake in Cambodia, a UNESCO Biosphere Reserve , is one of the world's most ecologically diverse lake ecosystems, home to endangered amphibians, reptiles, mammals and birds, with a diverse array of 885 species. It provides livelihoods for almost two million fishers, and its fish feed millions more.
But its future is in danger as the intensity of the unique 'reverse flow' that feeds water into the lake has been declining year-on-year.
Scientists have now conclusively determined the cause of this decline to be sand mining in the Mekong River in Cambodia and Vietnam.
Sand mining is the practice of extracting sand from the riverbed, with the majority used in the construction industry.
Rates of sand mining in the Mekong have rapidly increased, with over 100 million tonnes of sand now being removed every year.
Steve Darby, Professor of Physical Geography at the University of Southampton and co-author of the new research, said: "There has been lots of speculation as to why the intensity of the reverse flow has been declining, with climate change and damming on the Mekong upstream in China and Laos previously being identified as possible causes.
"Our work demonstrates that, while climate change and damming are minor contributory factors, by far the dominant driver has been riverbed incision caused by largely rampant sand mining on the Mekong."
The reverse flow system
Tonlé Sap Lake is sustained by an unusual reverse flow system. The Tonlé Sap River normally drains the lake, flowing downstream to Phnom Penh, where it joins the Mekong River.
But, during the monsoon season, the Mekong's flood pulse rises high enough to force the Tonlé Sap River to reverse for several months, filling the lake.
The Tonlé Sap Lake stores so much water during this seasonal flood pulse that it acts as a giant 'flood capacitor', regulating flood water levels down across the Mekong Delta – home to 23 million people – before releasing the stored freshwater back downstream to the Delta during the dry season.
The study, published in Nature Sustainability, shows that between 1998 and 2018, riverbed lowering of the Mekong River mainstem, driven by sand mining and upstream sediment trapping, has reduced the reverse flow volumes by between 40 and 50 per cent.
Projections to 2038, with additional riverbed lowering driven by ongoing sand mining, suggest that the reverse flow could decline by up to 69 per cent compared to 1998.
Dr Quan Le, Research Associate in Delta Flood Risk at Loughborough University and lead author of the study, said: "Rapid urban growth has fuelled a global surge in a demand for construction sands, increasing river sand mining rates.
"Our study finds that this intensive sand extraction, combined with sediment being trapped by dams in the Lower Mekong basin, has already weakened the Tonle Sap lake's flood pulse, causing lasting environmental harm and underlining an urgent need for sustainable sediment management to ensure that these future projections are not realised."
On average, riverbed levels across much of the Lower Mekong's course in Cambodia and Vietnam have dropped by two to three metres in the last two decades.
"At this rate, within 10 years the system is at risk of a near total collapse," said Professor Darby. "The Mekong is the second most biodiverse aquatic ecosystem in the world, after the Amazon, and its health depends on the normal functioning of the Tonlé Sap Lake. A collapse of the lake system would have catastrophic consequences for the biosphere, for millions of people's livelihoods and food sources, and for flooding in the region."
Understanding and mitigating the impact
Scientists are continuing to assess the consequences of sand mining in the region.
A project called Hidden Sands , led by Professor Julian Leyland at the University of Southampton, is investigating the impact of sand mining on the environment and on communities in Cambodia. The team has also been working with agencies in Vietnam to provide a more risk-based approach to the governance of sand mining.
Along with the above researchers, Craig Hutton, Professor of Sustainability Science and director of the Sustainability and Resilience Institute, and Paul Kemp, Professor of Ecological Engineering, both at the University of Southampton, are embarking on a new project to fully understand the ecological impact on Tonlé Sap Lake, particularly the impact on fish.
Professor Hutton said: "The lake is estimated to feed about six million people and provide as much as 60 per cent of Cambodia's protein. The disruption in lake levels from sand extraction along with deforestation, illegal fishing and extreme pesticide and fertilizer use are exacting a heavy toll on fish production. This decline is threatening both food security and livelihoods."
Professors Hutton and Kemp interviewed fishing communities in a recent visit to the region.
"Struggling fisher folk have seen as much as an 80 per cent mortality in aquaculture fish, declines in wild catch and mounting household debt," said Professor Hutton. "One interviewee told us, 'We just want another life for our children now. Anything but fishing'."
