A sediment plume extending hundreds of kilometers from shore is a prominent feature in winter months off the coast of China’s Jiangsu province. And much like the turbid water itself, the reasons for its occurrence are somewhat murky.

The VIIRS (Visible Infrared Imaging Radiometer Suite) sensor on the NOAA-20 satellite captured this image of sediments fanning out over the Yangtze Bank on February 12, 2024. Several rivers empty into the sea in this region, bringing abundant suspended material with them. The Yangtze River, the third longest river in the world, alone drains approximately 1.8 million square kilometers—an expanse equivalent to about one-fifth the area of China.

Brown, sediment-laden water is visible along this shallow, turbid stretch of coast year-round, but a vast plume like the one visible here is a characteristic feature of winter. Scientists have proposed several causes of this seasonal phenomenon.

The ebb and flow of tides contain enough energy to stir up sediments from the seafloor, a 2017 study found. While there are also tides in the summer, the models used in the study showed that sediment only rises to the surface in the winter. That’s when temperatures and salinities at the sea surface and bottom are similar, allowing for vertical mixing to occur and for sediment to churn higher into the water column.

Major currents in the East China Sea and Yellow Sea also change in intensity with the seasons. Other research has suggested that the summer monsoon’s influence on currents impedes the eastward flow of sediment out of the estuaries.

Despite what looks like a glut of suspended sediment in the water, the overall amount of it flowing from the Yangtze River has declined steadily over the past several decades. The construction of dams, including the Three Gorges Dam, has been the primary driver of this trend.

Researchers are interested in tracking the movement of suspended sediment because of its range of ecological impacts. For example, some contaminants stick to sediments and may harm organisms that live in or near those sediments. Suspended material also reduces the amount of light reaching aquatic ecosystems, curtailing photosynthesis and primary productivity.

NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission, launched in February 2024, will help scientists distinguish sediment from phytoplankton in coastal waters and better assess ocean health by providing hyperspectral observations of ocean color.

NASA Earth Observatory image by Wanmei Liang, using VIIRS data from NASA EOSDIS LANCE, GIBS/Worldview, and the Joint Polar Satellite System (JPSS). Story by Lindsey Doermann.