For the half billion people living on the world’s river deltas and the hundreds of millions of others who rely on them for water, food, shelter, transportation and energy, the news is not good.
More than two-thirds of the the world’s 33 major river deltas are sinking and the vast majority have experienced flooding in recent years, primarily a result of human activity, says CU-Boulder Professor James 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.
“These deltas are starved of the sediments they need for stability because of upstream dams that trap the material,” explains Syvitski, a professor in geological sciences who also is executive director of the international Community Surface Dynamics Modeling System (CSDMS). which is based in Boulder. “We are seeing coastal erosion increasing in many places across the planet.”
Unfortunately, deltas are sinking at a much greater rate than sea levels are rising, Syvitski notes. The Mississippi River Delta, for example, is losing a mind-blowing, football field-sized chunk of wetlands every hour. On the Yellow River Delta in China, a switch from agriculture to fish and shrimp farming -- which requires enormous amounts of groundwater -- is causing that delta to sink nearly a foot a year, he says.
“The rate of subsidence on the Yellow River is amazingly high -- the ground can sink 3 feet in 4 years and affect infrastructure like buildings and roads,” says Syvitski. “But more importantly, lowering the land surface makes it much more exposed to the ocean environment, including storm surges from hurricanes and tsunamis.”
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 notes. The Mississippi River system, for example, has more than 40,000 dams that are 20 feet or higher.
The Community Surface Dynamics Model System that Syvitski directs is a global, interdisciplinary program involving hundreds of researchers and students now in 500 institutes in 68 countries. Cross-disciplinary research groups develop integrated software modules that predict the movement of water, sediment and nutrients across landscapes and into the oceans. Major funding for CSDMS comes from the National Science Foundation.
“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,” Syvitski says. “The CSDMS effort gives us a better understanding of Earth and allows us to make better predictions about areas at risk to phenomena like flooding, deforestation, forest fires, land-use changes and the impacts of climate change.”
Photo: Mississippi River Delta from NASA’s Space Shuttle Discovery in 1985. Image courtesy NASA.