Quantifying the Geomorphic Effect of Floods Using Satellite Observations of River Mobility.

Geomorphologists have long debated the relative importance of disturbance magnitude, duration, and frequency in shaping landscapes. For river‐channel adjustment by floods, some argue that the cumulative hydrograph, rather than magnitude or duration, matters most. However, studies of flood‐induced river‐channel change often draw upon small data sets. Here, we combine Sentinel‐2 imagery with flow data from laterally active rivers to address this question using a larger data set. We apply automated algorithms in Google Earth Engine to map rivers and detect their lateral shifting; we generate a large data set to quantify planform erosion during 175 floods at 34 selected sites. Erosion during these floods is best explained by their duration and then their cumulative hydrograph. We use a random forest regression model to predict flood‐induced erosion, with potential applications for hazard management. Ultimately, better global data on sediment supply and caliber would help us to understand flood‐driven change to river planforms. Plain Language Summary: Some rivers change their shape over time. In this paper, we explore how high‐flow events drive these river channels to reshape themselves. We use Google Earth Engine to automatically map the shapes of rivers in satellite images. We apply this method to pairs of satellite images before and after high‐flow events, to understand how the river shape is changed by the event. We compare the amount of channel widening measured to aspects of the high‐flow event, including its peak, duration, and total flow. We do so for 175 events, and find that the duration and total flow are most important for explaining how much a channel widens during the event. Finally, we build a statistical model to predict the average amount of channel widening for a given high‐flow event. This method has potential applications for hazard management in rivers that are known to change their shape. Key Points: We develop a method to quantify river planform change after flood events, using Google Earth EngineWe do so for a data set of 175 floods that exceeded the 80th percentile stage, at 34 flow gauging sites on laterally active riversPlanform erosion during these high‐flow events was most correlated with the event duration, and then the summed hydrograph [ABSTRACT FROM AUTHOR]