Going down. Much of the Mississippi River delta in Louisiana has dropped below sea level (pink and red areas).


In Race With the Sea, River Deltas Falling Behind

River deltas are some of the most fertile and densely populated land on Earth. Unfortunately, many major deltas are sinking into the sea, a new satellite study confirms. Pin the blame on human activities such as oil and natural gas wells under the delta areas and dams upriver.

Deltas are built as rivers carry sediment downstream to the sea and deposit it at the mouth of the river. The new deposits can create land even though the underlying sediments and sea floor are sinking from geological processes. The fate of deltas is also influenced by sea level, which is rising from global warming. What's clear to residents of many of the world's largest river deltas--such as the Mississippi in the United States, the Indus in Pakistan, and the Po in Italy--is that they're facing rising waters from an increasingly lower vantage point.

To find out exactly how deep the trouble is, an international team used historical maps and other data, along with the latest satellite imagery and technology, to track elevation changes in 33 deltas over the past half-century. The researchers focused on the balance among river sediments, sea-level changes, and the sinking of the deltas. They also estimated natural subsidence rates based on existing field measurements. Using NASA's Shuttle Radar Topography Mission and other satellite images of sediment-rich floods, as well as computer models, the researchers compiled high-resolution maps, which they described online 20 September in Nature Geoscience.

The team found that 28 of the 33 deltas examined in the study have been sinking, on average, by 6.8 millimeter per year relative to sea level. And some, like the Po River delta in Italy, have dropped by as much as 60 millimeters per year.

In addition, the data show that the deltas are sinking four times faster than any natural process might account for, says geologist and co-author Irina Overeem of the University of Colorado, Boulder. Previous studies of delta areas, such as the Mississippi, had suggested that this phenomenon was occurring, "but we were struck both by the prevalence of this trend in deltas worldwide as well as by the rate," she says. One major factor behind the trend is gas and oil extraction, which increases the compaction of the deep sediments, so that the land sinks as gas and oil are removed. Dams also hurt deltas by catching sediment before it reaches the sea.

The maps represent a new and "holistic" way of looking at the deltas, says oceanographer Hartwig Kremer of Land-Ocean Interactions in the Coastal Zone, a research organization headquartered in Geesthacht, Germany. The study shows that human activity is currently driving deltas down much more than global warming is pushing sea levels up, he says, although that may change if the warming accelerates.