Landslides Limit the Pace of Bedrock River Incision in Southern Taiwan

Bedrock river incision is inhibited by sediment cover, which depends on the balance between sediment supply and transport. In steep landscapes, sediment supply is primarily controlled by mass wasting processes. Occasionally extreme events such as major storms, earthquakes, forest fires, and volcanic eruptions trigger many landslides at once, creating sediment pulses that overwhelm fluvial systems. Although many examples of landslides triggered by extreme events exist, their contribution to river and landscape evolution is not well understood. We document feedbacks between tectonics and sediment cover during and after extreme events and find that bedrock river incision is buffered preferentially in the most tectonically active areas by aggradation of landslide sediment. We quantify production, deposition, and transport of landslide sediments resulting from the 2009 Typhoon Morakot in Southern Taiwan, which delivered record-breaking rainfall triggering more than 22,000 landslides across 7800km². An annual landslide catalog allows use of area-volume scaling to estimate amount of landslide material distributed across a strong northward gradient in tectonic uplift in Southern Taiwan. Landslide volume and frequency exhibit similarly positive trends with distance from the southern tip of the orogen. Exploiting a wealth of publicly available imagery and elevation data, we map sediment aggradation throughout fifteen drainage basins and observe 10's of meters of aggradation with distribution tightly coupled to areas of greatest exhumation. Sediment transport modeling across the orogen suggests that areas of highest exhumation will be inundated with sediment over three orders of magnitude longer than less exhumed basins. Estimating the frequency of events like Typhoon Morakot, we expect the most active basins in the study area to have their channels buried by landslide sediment for up to 50% of any given time period, while less active basins will be able to incise nearly 100% of the same time period. This feedback suggests the presence of a threshold landscape steepness above which fluvial incision of bedrock slows following extreme events.