Human activity threatens world’s river deltas

U. COLORADO (US)—Of the world’s 33 major deltas, 24 are sinking, in large part due to human activity, and 85 percent experienced severe flooding in recent years, a new study finds.

The flooding resulted in the temporary submergence of roughly 100,000 square miles of land.  About 500 million people in the world live on river deltas.

“We argue that the world’s low-lying deltas are increasingly vulnerable to flooding, either from their feeding rivers or from ocean storms,” says study coauthor Albert Kettner, a research associate at University of Colorado at Boulder.  “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.”

The study is part of an effort called the Community Surface Dynamic Modeling System, or CSDMS, that involves hundreds of scientists from dozens of federal labs and universities around the nation.

The researches say human factors, including the upstream trapping of sediments by reservoirs and dams, man-made channels and levees that whisk sediment into the oceans beyond coastal floodplains, and the accelerated compacting of floodplain sediment caused by the extraction of groundwater and natural gas, are causing deltas to sink from Asia and India to the Americas.

The researchers  predict that global delta flooding could increase by 50 percent under current projections of about 18 inches in sea level rise by the end of the century as forecast by the 2007 Intergovernmental Panel on Climate Change report.

The flooding will increase even more if the capture of sediments upstream from deltas by reservoirs and other water diversion projects persists and prevents the growth and buffering of the deltas, according to the study.

Using satellite data from NASA’s Shuttle Radar Topography Mission, which swept more than 80 percent of Earth’s surface during a 12-day mission of the space shuttle Endeavour in 2000, the  researchers compared the SRTM data with historical maps published between 1760 and 1922.

“Every year, about 10 million people are being affected by storm surges,” says coauthor Irina Overeem. “Hurricane Katrina may be the best example that stands out in the United States, but flooding in the Asian deltas of Irrawaddy in Myanmar and the Ganges-Brahmaputra in India and Bangladesh have recently claimed thousands of lives as well.”

The researchers predict that similar disasters could potentially occur in the Pearl River delta in China and the Mekong River delta in Vietnam, where thousands of square miles are below sea level and the regions are hit by periodic typhoons.

“Although humans have largely mastered the everyday behavior of lowland rivers, they seem less able to deal with the fury of storm surges that can temporarily raise sea level by three to 10 meters (10 to 33 feet),” the authors say.  “It remains alarming how often deltas flood, whether from land or from sea, and the trend seems to be worsening.”

“We are interested in how landscapes and seascapes change over time, and how materials like water, sediments, and nutrients are transported from one place to another,” explains James Syvitski, professor of geological sciences.

“The CSDMS effort will give us a better understanding of Earth and allow us to make better predictions about areas at risk to phenomena like deforestation, forest fires, land-use changes, and the impacts of climate change.”

Researchers from Dartmouth College, Louisiana State University, City College of New York, the Geological Survey of Japan, Woods Hole Oceanographic Institute, and the University of Southampton in England contributed to the study, which was published in the Sept. 20 issue of Nature Geoscience.

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