Deltas are formed by sediments that rivers deposit at their mouths, and they house a tremendous amount of biodiversity. But around two-thirds of the world’s deltas are under threat from human activity. In the U.S., for example, the Mississippi River Delta is losing chunks of land the size of football fields to the sea every hour—with drilling, climate change, and levees built to stop the river from flooding all to blame for this phenomenon.
On the other side of the world, China’s Yellow River Delta has also seen dramatic changes in the last two decades, a new visualization created by Mapbox shows. In this viz (below), satellite images of the region from 1979, 1992, 1995, 2000, and 2015 flow in smooth succession over a timeline, showing the delta transformation over the last 35 or so years:
And here’s a GIF of that transformation Mapbox tweeted recently:
As it flows through the Loess Plateau, the Yellow River picks up 1.6 billion tons of silt a year—more than any other river on earth. All of this sediment gets deposited at the point where the river meets the Bohai Sea. This delta soil is incredibly fertile, but it also erodes easily. That means it tends to change course quite easily. Here’s NASA’s Earth Observatory describing some of the changes the delta has undergone as a consequence:
Between 1989 and 2000, astronauts on the Space Shuttle documented dramatic changes in the tip of the Yellow River delta. Over this time, several hundred square kilometers have accreted to and been eroded from the coast. The delta grew nearly 400 square km between 1989 and 1995, and then began eroding back. In 1997 a new channel was cut near the tip of the delta, providing the water and sediment a shorter route to the sea. Between 1995 and 1997, the delta area eroded back about 250 square km. From 1997 to February 2000, the delta tip again grew nearly 100 square km.
Human activity is not off the hook here, as Virginia Yung of Mapbox points out in a blog post. As in Louisiana, construction of dams, levees, and jetties, urbanization, and population growth definitely affected the Yellow River’s sediment-carrying capacity. Rice and fish farming practices, in particular, have caused swathes of land in the delta to sink, according to research at the University of Colorado-Boulder. Here’s one of the authors of that research, geologist James Syvitski, via press release:
“The rate of subsidence there is amazingly – 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.”