'Millions at risk' as deltas sink

Damming and diverting rivers means that much less sediment now reaches many delta areas, while extraction of gas and groundwater also lowers the land.

Rivers affected include the Colorado, Nile, Pearl, Rhone and Yangtze.

About half a billion people live in these regions, the researchers note in the journal Nature Geoscience.

They calculate that 85% of major deltas have seen severe flooding in recent years, and that the area of land vulnerable to flooding will increase by about 50% in the next 40 years as land sinks and climate change causes sea levels to rise.

"We argue that the world's low-lying deltas are increasingly vulnerable to flooding, either from their feeding rivers or from ocean storms," said Albert Kettner from the University of Colorado in Boulder, US.

"This study shows there are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone."

Most of the at-risk river basins are in the developing countries of Asia, but there are several in developed nations as well, including the Rhone in France and the Po in Italy.

The Po delta sank by 3.7m during the 20th Century, mainly from methane extraction, the researchers say.

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