Moderate flows help carve rivers

Measurements of erosion in a rocky river channel in Taiwan suggest that the day-to-day flow of water accounts for more rock wear there than occasional catastrophic floods do. The findings, which are contrary to current views, could revise scientists’ ideas of how rivers shape Earth’s surface.

In its 58-kilometer rush to the Pacific, the LiWu River picks up about 11 million tons of sediment each year. That material boosts the river’s scouring power, says Rudy L. Slingerland, a geologist at Pennsylvania State University in State College. He and his colleagues measured rates of erosion at more than 2,100 points across a short stretch of the rocky channel. Between December 2000 and December 2001, the river chewed down about 6 millimeters through quartzite rocks and about 2 mm through a tough type of rock known as schist.

But in the previous year, a supertyphoon pummeled the area and produced a flood in which the river’s flow rate peaked at about 65 times normal. From February to December 2000, the river stripped away about 82 mm of quartzite and 36 mm of schist. Even though those erosion rates are much greater than normal, such floods are expected only once every 20 years. Therefore, Slingerland notes, the long-term effects of infrequent catastrophic floods on the riverbed are just as important as the erosion produced by routine flow and small floods. The researchers report their analyses in the Sept. 20 Science.


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