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River shaping from floods happens in moderation

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An assessment of rivers in the US suggests that although there is a relationship between increased flood size and erosion, the effect is most pronounced for moderate floods. Understanding the effects of climate on the geometry and erosion rate of rivers is essential for reconstructing the geologic history of landscapes, as well as for predicting the response of rivers to human-accelerated climate change. Currently, models predict that climate should play a fundamental role in setting erosion rates, yet a strong correlation has not been verified in studies of bedrock erosion. To gain more insights into this relationship, Colin Phillips and Douglas Jerolmack first monitored cobble displacement in the Mameyes River in northeastern Puerto Rico, a site that frequently experiences flash floods; based on this data collection, they established the critical threshold for sediment displacement. Next, the team used this baseline to analyze channel geometry and stream-flow records from 186 coarse-grained rivers across the United States. Their analysis reveals that river channel sculpting does increase as flood size increases, but this effect is most pronounced for moderate floods; river channels actually adjust to the conditions of major floods so that larger floods have very limited additional impact on channel geometry. This in turn "blunts" the impact of extreme rainfall events on landscape evolution. The results may explain the long-term stability of rivers across long-term shifts in climate.

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