The majority of the world’s river deltas are sinking due to human interference, increasing the flood risk of as many as half a billion people, according to a new study published in Nature Geoscience.

Researchers at the University of Colorado combined data from space missions and satellite programmes with historical records and measurements of sea-level rise to calculate whether land is sinking in 33 major river deltas and to try to determine what factors might be responsible. They found that 24 of the deltas were sinking, including those of the Rhone in France, the Nile in Egypt and the Pearl, Yangtze and Yellow in China, while those in five regions, including the Amazon and the Congo, were stable.

Among the worst affected was the Chao Phraya, which flows through Bangkok. The researchers found that in some years, parts of the delta sank by as much as 15 centimetres relative to sea level.

In many cases, the sinking is the result of damming and diversion, which stops sediment from reaching the delta. The extraction of gas and groundwater can also cause the land to drop. The Po delta in Italy sank by 3.7 metres during the 20th century, primarily as a result of methane extraction.

The researchers calculated that 85 per cent of the world’s major river deltas have experienced severe flooding in recent years. Over the next 40 years, they say, further sinking will see 50 per cent more land become vulnerable to floods.

December 2009