“I would like for me and my children to live here forever,” said Lâm Thu Sang, a resident of Vietnam’s Cần Thơ, a city of more than 2 million people located near the mouth of the Mekong River on one of the world’s largest river deltas.
But that may not be possible.
In the past, about 160 million metric tons of sediment was annually funnelled down the 4,300-kilometre (nearly 2,700-mile) Mekong River to form and nourish the vast delta where the river meets the sea. By 2024, that deposition rate had fallen by 70 per cent per year — starving the delta of much of its source material.
The Mekong flows through six Asian nations, draining a roughly 800,000-square-kilometre (309,000-square-mile) basin, until finally releasing its combined sediments into the 40,000-km2 (15,400-mi2) Mekong Delta — a complex ecological system of low-lying fertile lands and a web of waterways the size of the Netherlands, stretching from Phnom Penh, Cambodia, to the South China Sea in Vietnam.
Unfortunately, the future of Lâm Thu Sang’s community and this great delta are seriously in doubt, with the delta doubly threatened by land subsidence and sea level rise.
Sang, who helps run the Anh Duong Community Development and Support Center, an NGO focused on eradicating poverty in remote areas of Cần Thơ, said that people know their delta home is sinking, and, in a WhatsApp video interview with Mongabay, acknowledged that local communities are already living with the consequences, including increasingly devastating floods.
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Some people say, maybe in about 50 years, there will be no more Mekong Delta on this Earth, no more us.
Lâm Thu Sang, campaigner, Anh Duong Community Development and Support Center
“Some people say we will have to move to the centre [of Vietnam],” Sang said.
But this isn’t just an Asian crisis.
The Mekong is just one of the world’s mega-river deltas that support vast tracts of rich agricultural land, productive fisheries, and intense urban development. These deltas are sinking beneath the feet of the people living on them, and may be lost this century should the world fail to heed the mounting evidence of an already unfolding disaster.
The ‘double burden’ borne by today’s deltas
A study published in Nature in January 2026 used satellite radar data to map vertical land movement at high resolution and identified elevation loss (much of it driven by human activity) in 40 of the world’s largest deltas occurring between 2014 and 2023. They detected some of the most rapid, widespread land subsidence in 19 deltas, including the Mekong, Nile, Chao Phraya, Ganga-Brahmaputra, and Mississippi rivers.
“It’s one of the first high-resolution, near-global assessments of delta subsidence at this scale,” study lead author Leonard Ohenhen, an assistant professor in Earth system science at the University of California, Irvine, told Mongabay in a Zoom interview.
The research points to a consistent, underrecognised, and alarming pattern: The world’s deltas are subject to a “double burden” of rapidly rising sea levels due to climate change, along with sinking land — subsidence driven by local processes including aquifer pumping for drinking water and agriculture, along with impacts from upstream dams that greatly reduce water and sediment flows.
As deltas sink, the world’s oceans are rising to meet them. Together, these forces result in relative sea level rise at rates that often exceed global averages.
“We discovered that land subsidence is a huge contributor to relative sea level rise, but it’s often overlooked,” Ohenhen said. “Sea level rise is mentioned thousands of times in major [climate] reports, while vertical land motion [due to subsidence] is barely discussed, even though it can dominate the risk in many coastal settings.”
Across more than half of the deltas studied, land is sinking faster than 3 millimetres (0.12 inches) per year. In at least 13 deltas, including the Nile, Mekong, Chao Phraya, Brantas and Yellow rivers, average subsidence rates exceed the already alarming current global annual sea level rise of around 4 mm (0.16 in). The Chao Phraya, Brantas and Yellow river deltas, for example — in Thailand, Indonesia and China, respectively — are sinking at an average rate of more than twice that.
In some places, the imbalance created by land subsidence and sea level rise is far greater. In parts of the Indo-Gangetic Plain and within several Asian deltas, some localised areas are sinking at rates up to 20 times faster than sea level rise.
Seven of the world’s largest deltas, including the Mekong, Nile and Ganga–Brahmaputra, account for more than half of all subsiding delta area globally, covering a combined surface area roughly the size of Aotearoa New Zealand.
Within these and many other delta systems, the majority of existing land is already impacted. About 80 per cent of the Nile Delta’s area is subsiding at rates of roughly 5 mm (0.2 in) annually. In the Chao Phraya Delta, the figure rises to 94 per cent. In the Mekong, it exceeds 50 per cent.
And it’s not only critically important croplands and fisheries at risk. Major cities built on these deltas, including Alexandria, Bangkok, Shanghai and Ho Chi Minh City, are sinking at rates equal to or greater than surrounding landscapes.
Planetary ramifications
The significance of this ongoing loss extends far beyond the deltas themselves. Flat, fertile, water-rich deltas nurtured the cradles of human civilisation on the Nile, Indus, Yangtze, Yellow and Tigris-Euphrates rivers, and supported farming for thousands of years.
Today, they remain among the most densely populated and agriculturally productive landscapes on Earth. Despite covering just 0.5 per cent of the planet’s land surface, deltas produce around 4 per cent of the world’s food.
In some countries, their role is absolute. The Nile Valley and delta occupy just a fraction of Egypt’s land, yet support most of its population and nearly all its agriculture, making it one of the most concentrated food production systems on Earth.
The Mekong Delta plays a similar role at a different scale. Often referred to as Vietnam’s “rice bowl,” it connects around 1.5 million farmers to domestic and international markets. The delta region produces more than half of Vietnam’s staple crops, around 65 per cent of its aquaculture output, and roughly 70 per cent of its fruit. It also contributes about a third of Vietnam’s agricultural GDP and supports roughly 20 million people.
The Mekong Delta’s influence extends far beyond national borders. The 25 million metric tons of rice it produces annually helps feed the world and allows Vietnam to become one of the planet’s leading rice exporters, contributing significantly to global food security.
The world’s deltas are also home to rapidly growing human populations. Around 680 million people live in low-lying coastal zones and river deltas, a number projected to increase to 1 billion people by 2050. If coastal land loss is not curbed, it could trigger human migration and a sociopolitical crisis at a staggering scale.
Human pressures, sinking deltas
The Nature study not only detected delta subsidence; its researchers also pinpointed the drivers, including reduced sediment delivery, urban expansion, and, in particular, groundwater extraction.
In the Mekong, these pressures converge. More than half the sediment that once reached the delta is now trapped in reservoirs behind dams. Currently, 745 dams are complete or under construction on the Mekong mainstream and its basin tributaries. Existing and proposed dams (if all completed) could trap up to 96 per cent of the sediment that historically nourished the delta.
Marc Goichot, freshwater lead for the Asia-Pacific region at WWF, noted in a WhatsApp video call with Mongabay that all sediments are not created equal, and the devil is in the delta-forming details. Nutrient-rich fine sediment (silt and clay that form mud) occurs in large quantities along rivers. In comparison, coarse sediment (sand and gravel), which constitutes only about 15 per cent of the total sediment load, plays a disproportionate role in delta land formation and physical integrity, but can’t pass through dams.
As such, he said, while total sediment load reduction due to dams is heading to more than 90 per cent in the coming decades, sand sediment reduction is already largely negative, dwindling to an annual 3 million to 4 million metric tons down from about 20 million to 30 million metric tons per year in 1990.
In addition, sand mining removes an estimated 54 million metric tons of sediment from the river each year, primarily for construction, though Goichot said that, basin-wide, it is likely double that amount. Sand extraction deepens river channels, accelerates erosion, and further reduces the flow of sediment to build up land in the race to keep the delta above sea level.
“The river is now two to three meters [6-10 feet] deeper than 20 years ago, but you don’t see it because you only see the water surface,” Goichot said. The delta’s sand budget is in a big deficit, he noted: “We are eating the stock, instead of living on the annual income.”
Together, these human processes are creating a gross imbalance that one day will no longer allow the delta to persist.
Delta loss: ‘A policy choice, not a natural fate’
The implications of the delta decline are clear for Jeff Opperman, WWF’s global lead scientist for freshwater. The Mekong Delta, for example, is not simply a landscape at risk, he told Mongabay in a Zoom interview. It is a vital living system that supports tens of millions of people, a large share of Vietnam’s national economy, and a significant portion of the global rice trade.
“If current trends continue,” he said, “around 90 per cent of the Mekong Delta would be underwater by the end of this century.”
However, this same modelling points to a positive alternative trajectory. If groundwater extraction is reduced, sand mining controlled, and sediment losses minimised, subsidence could be limited to around 150 mm (6 in) in total, with far less land lost.
“We’re not just talking about a local wetland going under,” Opperman said. “We’re talking about a food-producing engine for Vietnam and the world. Losing most of this delta is a policy choice, not a natural fate.”
A system that can be rebuilt
Unlike sea level rise, which will continue for decades even under strong climate action, subsidence responds more directly to human decisions. “It can be slowed or even stopped,” Ohenhen said, pointing to solutions that include reducing groundwater extraction, managing land use, and restoring sediment flows.
Goichot said that the only way to truly counter subsidence and build elevation against rising seas is to allow flooding to add a new layer of sediment and address the root causes of delta loss rather than aggravating factors. Rebuilding sediment delivery from the river to the Mekong Delta’s floodplains is the only solution, he said. “There are no others,” sedimentation being the process that built the delta in the first place.
Opperman said that policy decisions need to distinguish between what rivers and deltas provide of value that can be replaced and what cannot. The Mekong now supports one of the world’s most productive inland fisheries and one of its most diverse freshwater ecosystems, he said, and its delta underpins major shares of global agricultural production. Both depend on connectivity and uninterrupted movement of water, sediment, and species along the river.
“These are not services you can simply rebuild once they’re gone,” he said. “If you starve a delta of sediment and it slips under the sea, you can’t just send trucks of sand and expect to get the same system back.”
By contrast, he noted that the benefits that have driven much of the river’s recent transformation, particularly hydropower dams, are increasingly replaceable as alternative energy sources expand.
Addressing subsidence requires coordination across sectors and scales, but for Goichot, at the core is a deeper economic blind spot: Rivers are still treated primarily as sources of extractable resources — water, sand, energy — while their system functions remain largely invisible and ignored. “We need to value the river’s intangibles, beyond water as a commodity.” Geomorphological stability of riverbanks, flood dynamics, sediment flows, and delta elevation should be treated as assets in business plans rather than externalities, he said.
“Deltas only function if we recognise them as dynamic systems,” he said. “A dynamic equilibrium is more stable than something rigid and fixed.”
He pointed to historic floodplain societies that adapted to seasonal flooding and used it to sustain agriculture and fisheries. Traditional societies living along the banks of the Mekong built livelihoods in a symbiotic manner working with floods, Goichot said. “They saw floods as beneficial rather than destructive.” But modern riverine cultures can no longer cope.
“Now we are losing both the benefits of floods and our resilience to them.”
A narrow intervention window
Ohenhen suggested that policymakers need look no further than the Ciliwung Delta — on which Jakarta, Indonesia’s capital and one the world’s biggest cities, is built — to see the threat posed by the delta subsidence crisis. Often described as the fastest sinking city in the world, Jakarta has extracted vast amounts of freshwater from the unstable alluvial soils of its delta aquifers to provide water for its millions of residents.
The city is now subsiding at an estimated average rate of 50-60 mm (2-2.4 in) a year, even as oceans rise — and Jakarta is far from alone on the island of Java. A study published in Science Advances in April 2026 found that subsidence-driven flooding threatens the island’s entire northern coastline, home to a chain of coastal cities that will be inundated far sooner than climate models alone would predict. In response to regular devastating flooding, Indonesia is already relocating its capital to Nusantara, a new city being built inland on the island of Borneo.
On the Mekong Delta, life has become more precarious. Sang said floods now last far longer, with higher water levels than before, with muddy water frequently pushing into people’s homes, and even beds, and disrupting daily life.
Farmers are unable to grow rice or vegetables or raise animals during flood periods, while small traders and restaurant owners are forced to close. Sang’s NGO now offers emergency food support and cash so families can repair damage and restart livelihoods. But that’s a stopgap.
Groundwater that communities once relied on continues diminishing, prompting government restrictions on well use, she said, but farmers don’t have enough money to buy land elsewhere.
“Some people say, maybe in about 50 years, there will be no more Mekong Delta on this Earth, no more us.” The outcome for the delta and its people, Sang added, remains unknown.
This story was published with permission from Mongabay.com.
