A River Runs Over It

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Science  31 Mar 2000:
Vol. 287, Issue 5462, pp. 2377
DOI: 10.1126/science.287.5462.2377a

Many waterfalls are located where a river runs over hard rocks, and large-scale geologic features such as faults influence the courses of many rivers. But how important are small-scale features in the bedrock in controlling how erosion occurs?

Whipple et al. examined this question, which is central in understanding how landscapes evolve, in a variety of river settings. Where there were many cracks in the bedrock, erosion occurred mostly by movement of the bedload: small clasts and pebbles became lodged in cracks and were then jostled or knocked, plucking out joint blocks from the bed. In massive rocks, erosion was produced mostly by abrasion from sand suspended in the flow, not by particles bouncing or sliding along the river bed. A surprising suggestion is that cavitation, the formation and collapse of bubbles in the flow, may contribute by loosening grains in rocks and by initiating the formation of potholes in massive rocks.—BH

Geol. Soc. Am. Bull.112, 490 (2000).

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