Global Study Warns of Rapid Subsidence in River Deltas

River deltas are low-lying landforms that form when rivers deposit sediment into larger bodies of water, such as oceans or lakes.

River deltas are dynamic landforms formed by sediment deposited as rivers flow into larger bodies of water, such as oceans or lakes. These regions are crucial for biodiversity and provide essential habitats, making them vital for both the environment and human communities.

Although they account for just 1% of the world’s land area, they are home to up to 500 million people—around 6% of the global population. Notably, 10 of the world’s 34 megacities are located in delta regions.

Deltas play critical economic, environmental, and social roles. They support agriculture and fisheries, provide habitats for diverse species, and host essential infrastructure such as ports that underpin global trade.

Sinking land and rising seas

Despite their importance, many of the world’s major river deltas are under threat. These densely populated and economically vital regions are increasingly vulnerable to flooding, land loss, and saltwater intrusion.

A key factor exacerbating these risks is land subsidence. As delta land sinks—often simultaneously with global sea-level rise of around 4 millimetres per year—the combined effect significantly heightens flood risk.

A collaborative international study published in Nature in January 2026, titled “Global subsidence of river deltas, found that many deltas are sinking faster than sea levels are rising, placing millions of people at risk.

What the research reveals

The study used Interferometric Synthetic Aperture Radar (InSAR), a satellite-based remote sensing technique that measures minute changes in the Earth’s surface, to map elevation changes across 50 river deltas worldwide. This provided the first high-resolution global dataset on delta subsidence.

The findings are concerning. All deltas analysed showed some degree of sinking. In 12 out of 40 deltas, subsidence rates were moderate, at below 2 mm per year. However, more than half recorded rates exceeding 3 mm per year.

In 13 deltas—including the Nile, Mekong, Ganges–Brahmaputra, and Yellow River—the average subsidence rate exceeds the global sea-level rise of approximately 4 mm per year. In some cases, such as the Chao Phraya (Thailand), Brantas (Indonesia), and Yellow River (China), sinking rates are more than double this level.

Furthermore, in 18 of the 40 deltas studied, subsidence already surpasses local sea-level rise, increasing flood risk for more than 236 million people in the near future.

Overall, the study estimates that approximately 460,370 km² of delta land is affected by subsidence. The greatest exposure is found in South, East, and Southeast Asia, where 17 major deltas account for around 274,000 km² of sinking land.

Human activities accelerating subsidence

While deltas naturally subside over time due to sediment compaction and tectonic processes, human activities are significantly accelerating this process.

The study identifies groundwater over-extraction as a major driver of subsidence in at least 10 of the deltas analysed. Other contributing factors include oil and gas extraction, urban expansion, and agricultural land-use changes.

These activities are effectively turning a slow, natural geological process into a rapid and potentially hazardous environmental issue.

As co-author Manoochehr Shirzaei notes, “Subsidence is not a distant future problem—it is happening now, often at rates that exceed climate-driven sea-level rise.”