Extreme Flood Sediment Production and Export Controlled by Reach-Scale Morphology

Rapid earth surface evolution is discrete in nature, with short-duration extreme events having a widespread impact on landscapes despite occurring relatively infrequently. Here, we exploit a unique opportunity to identify the broad, process-based, controls on sediment production and export during extreme rainfall-runoff events through a multi-catchment analysis. A 3 hr extreme rainfall event generated significantly different impacts across three catchments, ranging from (a) sediment export exceeding two orders of magnitude more than the typical long term average to (b) a minimal impact, with this variability primarily controlled by catchment steepness and the presence of reach-scale morphological transitions caused by postglacial landscape adjustment. In any catchment worldwide where populations are at risk, we highlight the importance of combining topographic analysis with detailed mapping of channel bed material (e.g., presence of transitions between process domains) and identification of sediment sources within morphological transition zones for accurately predicting the impact of extreme events.