Deltas sink worldwide, increasing flood risk.

• Author: Block, Ben
• Position: EYE ON EARTH - Brief article

Most of the world's major river deltas are sinking, increasing their vulnerability to severe storms and floods, new satellite studies find. Of the world's 33 major river deltas, 24 are sinking due to flood-control efforts and other human-caused changes to river systems. The study, published in the September 2009 issue of Nature Geo-science, "conservatively" estimates that the area vulnerable to flooding could increase by 50 percent worldwide.

An estimated 500 million people live on river deltas, which are created as sediments are deposited at the mouths of rivers. Seasonal floods typically expand deltas, but flood-control efforts such as levees and dams have prevented regeneration in many delta systems. Deltas are also being tapped for water and mineral resources, causing the ground to subside faster than new sediments arrive.

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In the past decade, 85 percent...