Most of the world's river deltas are sinking relative to sea level, putting millions at risk of severe flooding, say researchers.
And human activity, such as dam-building, is the biggest reason, according to the study published last week in Nature Geoscience (20 September).
The researchers found that 85 per cent of the major river deltas studied experienced severe flooding in the past decade.
The area of deltas vulnerable to flooding will increase by half if sea levels rise by an average of 44 centimetres this century as projected.
They studied 33 river deltas, formed by deposits of sediment where a river joins another body of water such as an ocean, by combining satellite and remote sensing data with historical maps to determine their height relative to sea level, and looked at flooding events and sediment deposits.
They found that 26 deltas were sinking relative to sea level.
"A delta is a dynamic system, so a natural delta is sinking anyway," Robert Nicholls, professor of coastal engineering at the United Kingdom's University of Southampton and one of the authors of the study, told SciDev.Net. "But every flood season, sediment is brought back in and maintains the level of the land."
But in many rivers, sediment upstream of the deltas is getting trapped in reservoirs and dams, and the removal of oil, gas and groundwater is compacting the sediment.
Large areas of delta are less than 2 metres above sea level, making them vulnerable to flooding, particularly from tropical storm surges, which can temporarily raise sea levels by 3–10 metres.
For example, in 2008 a 3.5-metre storm surge caused by cyclone Nargis flooded the Irrawaddy Delta in Myanmar, killing thousands (see Ignored warnings 'worsened' Myanmar cyclone disaster).
Nicholls says that the banks of many river deltas — particularly in South and South-East Asia — are densely populated. The Pearl Delta in China and the Mekong Delta in Vietnam, both home to millions, are particularly at risk of flooding.
Many developing countries have yet to address the issue.
"You cannot stop deltas from sinking, but you can reduce how much they sink, and developing countries need public funding and coordinated programmes [to prevent major consequences from the floodings]," says Nicholls. "But they have to start by recognising the problem."