Sinking river deltas put millions at risk of flooding

The world’s most economically and environmentally important river deltas are sinking, putting millions of people at risk of flooding. In most cases, the subsidence of river deltas poses a greater threat to the communities living there than sea level rise, according to an analysis of satellite data.

Up to half a billion humans live in river deltas, including some of the poorest populations on the planet. Ten megacities with more than 10 million inhabitants are located in these vast low-lying areas.

Manoochehr Shirzaei of Virginia Tech and his colleagues attempted to determine the speed at which 40 river deltas flow around the world, including the Mekong, Mississippi, Amazon, Zambezi, Yangtze and Nile.

According to Shirzaei, the subsidence causes a double whammy in terms of flooding because, at the same time as the deltas are sinking, sea levels are rising by about 4 millimeters per year.

The team used data from 2014 to 2023 obtained by the radar of the European Space Agency’s Sentinel 1 satellite, which can measure changes in distance between the satellite and the ground with an accuracy of 0.5 mm. In all 40 deltas, more than a third of each area is sinking, while in 38 of the 40, more than half of the area is sinking.

“In many cases, land subsidence is a more important driver of relative sea level rise than the ocean itself,” says Shirzaei. “Average subsidence exceeds sea level rise in 18 of the 40 deltas, and dominance is even stronger in the lowest areas, less than a meter above sea level.”

Thailand’s Chao Phraya Delta, where Bangkok is located, is the worst of the 40 in terms of sinking rate and area affected. Its average subsidence rate is 8 mm per year – twice the current global average sea level rise – and 94 percent of the delta area is sinking faster than 5 mm per year.

The combined effect of sinking land and rising seas means that Bangkok and the Chao Phraya Delta are facing sea level rise at a rate of 12.3 millimeters per year. Alexandria in Egypt and the Indonesian cities of Jakarta and Surabaya also face rapid subsidence.

The team also examined data on three major human pressures – groundwater extraction, sediment alteration and urban expansion – to determine which had the greatest impact on delta subsidence. Dams, levees and upstream river engineering can reduce the supply of sediment that would otherwise help deltas build or maintain elevation, Shirzaei says. At the same time, urban expansion puts increased pressure on the delta surface and often increases water demand, which can indirectly intensify groundwater depletion.

Of these factors, groundwater extraction has the strongest overall influence, but some deltas are more influenced by sedimentary changes and urban expansion, the researchers found.

Shirzaei believes it is a mistake for policymakers to focus solely on sea level rise caused by climate change, which risks distracting adaptation efforts. “Unlike global sea level rise, human-caused subsidence can often be addressed locally through groundwater regulation, aquifer recharge management, and sediment management,” he explains.

Data centers, which use large amounts of water for cooling, could contribute to the problem, Shirzaei believes. “Our study shows that groundwater extraction is the main driver of rapid land subsidence in many river deltas, and that water-intensive facilities such as data centers can compound this risk if they rely on local water supplies,” he says.

In already vulnerable regions like the Mekong Delta, increased demand for water can accelerate land subsidence, undermine drainage and flood protection systems, and shorten the lifespan of critical infrastructure. “This doesn’t mean that data centers should never be built on deltas, but it does mean that they should avoid using groundwater, minimize water demand, and explicitly account for subsidence,” says Shirzaei.