By Richard Black
Environment correspondent, BBC News website
Damming rivers has slowed the flow of sediment that would replenish land
Most of the world's major river deltas are sinking, increasing the flood risk faced by hundreds of millions of people, scientists report.
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.
The researchers drew on data from various space missions including the Shuttle Radar Topography Mission, an 11-day project run from the shuttle Endeavour in 2000, and the Moderate Resolution Imaging Spectroradiometers (Modis) carried on two Nasa satellites.
Combined with historical records and measurements of sea level rise, this allowed the team to view how fast land was sinking in some deltas, and to look at the various factors that might be responsible.
Of the 33 major deltas studied, 24 were found to be sinking.
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Deltas with "virtually no aggradation (supply of sediment) and/or very high accelerated compaction"
Chao Phraya, Thailand
Sao Francisco, Brazil
Possibly the worst affected is the Chao Phraya, the river that flows through Bangkok. In some years, parts of the delta have sunk relative to sea level by 15cm (six inches).
This is significantly more than the global rate of sea level rise as a consequence of climate change (1.8-3.0mm per year).
The flow of sediment down to the Chao Phraya delta has been almost entirely blocked, the researchers report - by taking water out for irrigation, damming the river, and directing the main flow through just a few channels.
Normally, this sediment would add to the height of the land, a process known as aggradation.
Taking water from aquifers for drinking, industry and agriculture is also compacting the ground, making it sink.
As the ground falls and sea level rises, people become more vulnerable to inundation during storms.
"Every year, about 10 million people are being affected by storm surges," said Irina Overeem, another of the study team from the University of Colorado.
"Hurricane Katrina may be the best example in the US, but flooding in the Asian deltas of the Irrawaddy in Burma and the Ganges-Brahmaputra in India and Bangladesh have recently claimed thousands of lives as well."
The team identifies the Mekong and the Pearl River delta near Hong Kong as places where similar disasters are likely in future.