World's major deltas are shrinking: study

More than two-thirds of the world's 33 major deltas are sinking and the vast majority of those have experienced flooding in recent years, primarily a result of human activity, a new study has found.
From the Yellow River in China to the Mississippi River in US, researchers are racing to better understand and mitigate the degradation of some of the world's most important river deltas, according to Professor James Syvitski from the University of Colorado Boulder.
Researchers found that over two-thirds of the the world's 33 major deltas are sinking and the vast majority of those have experienced flooding in recent years, primarily a result of human activity.

Some 500 million people live on river deltas around the world, a number that continues to climb as the population increases, they said.
"These deltas are starved of the sediments they need for stability because of upstream dams that trap the material. We are seeing coastal erosion increasing in many places across the planet," said Syvitski.
River deltas are land areas created by sediment that collects at the mouths of rivers as they enter slow-moving or standing water like oceans and estuaries.

Human effects on river deltas range from engineering tributaries and river channels, extracting groundwater and fossil fuels, trapping sediments behind dams, reducing peak flows of rivers and varied agricultural practices, he said.

"Deltas are sinking at a much greater rate than sea levels are rising," Syvitski said.
One positive action was taken on the 3,395-mile-long Yellow River recently when some major dams were flushed of their sediments and sent rushing downstream, said Syvitski.
"This might be the first time that dam operators on the Yellow River have worked with people in the coastal zone to solve a problem," he said.
But looming threats to the Yellow River Delta include the sinking, or subsidence of land, caused in large part by a move from rice farming to aquaculture - raising fish and shrimp, he said. The land in some areas there is sinking by 10 inches per year as groundwater is pumped to the surface.

"The rate of subsidence there is amazing - the ground can sink 3 feet in four years and affect infrastructure like buildings and roads," Syvitski said.
"But more importantly, lowering the land surface makes it much more exposed to the ocean environment, including storm surges from hurricanes and tsunamis," he added.