Extreme Rainfall Doesn’t Always Mean Extreme Erosion

In the Puerto Rican rain forest, a strong storm can drop a meter of rain in a single day. All that water rushes into mountain rivers and causes a torrent as the water overflows the riverbanks and charges downstream. It seems intuitive that the force of so much water would lead to massive erosion of a riverbed. But according to a new study, that intuition is incorrect.

The work, published in the journal Science, shows that, though extreme precipitation events can greatly increase the amount of water traveling through a river, large storms only move about 50 percent more sediment than a typical rainfall. The overall contribution of these intense rainfalls to erosion, therefore, is smaller than might be expected.

With climate change expected to bring more intense precipitation in many regions of the globe, the findings indicate that, while these extreme rainfall events may at first lead to more flooding, river channels may rapidly increase their size to accommodate the flow.

“Our work suggests that river channels may set the speed limit on erosion,” said Douglas J. Jerolmack, associate professor of earth and environmental science. “We showed that the forces of the biggest flood events were really only incrementally larger than the moderate events because river channels adjust their size to be close to the so-called ‘threshold of motion,’ or the force required to move particles on the riverbed.”

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