The coastal waters along China’s Jiangsu province are brown all year round due to the large volume of suspended sediment that flows out from the Yangtze, Yellow, and other rivers.

But every winter, an even larger tongue of sediment emerges over the Great Yangtze Bank and extends hundreds of kilometers into the East China Sea. These winter plumes are prominent features in satellite imagery for a few months, before fading away in the spring. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image of a plume over the Great Yangtze Bank on November 9, 2017.

Remote sensing scientists find the feature perplexing and have advanced several theories about the causes. Some have argued that the plume is a product of currents moving sediment-laden river water eastward from the coast. Others have argued that it is caused by tides lifting up sediment that was deposited on the bottom of the Great Yangtze Bank hundreds of years ago.

A new study in the Journal of Geophysical Research Oceans makes the case for the latter option. After gathering data on waves, sediments, and currents as observed in January 2016 (when the sediment plume was visible in satellite imagery), researchers developed a model that simulated conditions in this part of the ocean. They ran a series of computational experiments that showed that the energy of tides is strong enough to stir up bottom sediment from the Yangtze Bank.

The tides do this all year round, the scientists think, but their modeling shows that the sediment can only rise up to the surface in the winter, when temperatures and salinities at the sea surface and bottom are roughly the same. In the summer, an influx of fresh water from the Yangtze, combined with heating of the surface layers of the sea, prevents vertical mixing and keeps the resuspended sediment in the depths.