The world's 33 major deltas are sinking and the vast majority of those have experienced flooding in recent years, primarily as a result of human activity, says a new study.

Some 500 million people live on river deltas around the world, a number that continues to climb as the population increases, the study pointed out.

"These deltas are starved of the sediments they need for stability because of upstream dams that trap the material," said researcher James Syvitski, professor in geological sciences at the University of Colorado Boulder in the US.

"We are seeing coastal erosion increasing in many places across the planet," Syvitski noted.

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.

The findings were presented at the 2016 Ocean Sciences meeting held in New Orleans, US.

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.

"Deltas are sinking at a much greater rate than sea levels are rising," Syvitski said.

The findings are based on international Community Surface Dynamics Modeling System (CSDMS) - global, interdisciplinary programme involving hundreds of researchers and students in 500 institutes in 68 countries.

For the study, the researchers looked into the degradation of major river deltas in the world, from Yellow River in China to the Mississippi River in Louisiana, US.

The two major river deltas in the United States are the Mississippi River Delta in Louisiana and the Sacramento-San Joaquin River Delta in California.

While the Sacramento-Joaquin Delta has significant issues with agricultural, industrial and urban pollution and subsidence, things are more dire in the Mississippi River Delta, where a football field-sized chunk of wetlands disappears every hour, Syvitski said.

There are more than 40,000 dams 20 feet or higher on the Mississippi River system, he noted.