River deltas are sinking due to human activity

Some of the most fertile farm fields and highly populated areas in the world are around large river deltas. Now, a new study led by James Syvitski, and colleagues from six other institutions, have shown that river deltas around the world are sinking due to human activity, making them increasingly vulnerable to flooding from ocean storms and putting tens of millions of people at risk. The scientists in the study, took a global look and compiled data from high resolution satellite images, historical records and maps, look at computer models and calculated how much sediment is flowing down by the rivers.

They examined 33 deltas around the world and found that 24 of those are sinking and that 85 percent experienced severe flooding in recent years, resulting in the temporary submergence of 260,000 km2.

In a sense, deltas always sink, because when a major river empties its water out to the sea, it also carries sediments which are deposited near to the mouth of the river, building a bank of sand that reaches the edges of water. As the sediments area in the delta increases, it eventually reaches the sea-level, while at the same time, the sea floor is subsiding and the sediments themselves under their own weight are compacting.

But with global warming, sea level are changing; as glaciers are melting, more water is entering the oceans, sea-levels go up, and as a result deltas tend to get flooded more often. In addition to global warming, researchers discovered that humans are having two more impacts, locally, to deltas. Drilling and extraction of oil, natural gas and water, exacerbate the compaction rate of the sediments and the sinking of the delta.

The second impact is coming from a bit further upstream. Man-made dams and reservoirs hold back the water of the river but also trap the sediments, the same sediments that the river uses to build up its delta.

The scientists estimate that the delta surface area vulnerable to flooding could increase by 50 percent under the current projected values for sea-level rise in the 21st century. This figure could increase if the capture of sediment upstream persists and continues to prevent the growth and buffering of the deltas.

Source: Nature Geoscience 2, 681 – 686 (2009) Published online: 20 September 2009 doi:10.1038/ngeo629. Image Credit: geology.com

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