Sediment transport length scales shape the tools-and-cover effect on bedrock incision

In mountain rivers, sediment from landslides or debris flows can alluviate portions or even full reaches of bedrock channel beds, influencing bedrock river incision rates through the “tools and cover” effect. Various landscape evolution models have been developed to account for the coevolution of alluvial cover and sediment-flux-dependent bedrock incision. Despite the commonality of their aims, one major difference between these models is the way they account for sediment transport length scales. Sediment can be transported over long distances before it settles, and sediment flux depends not only on local topographic and hydraulic conditions, but also on non-local upstream sediment supply and flow conditions. Here, we compare numerical simulations of sediment transport and sediment-driven bedrock abrasion across a range of sediment transport length scales. Our results show that longer transport length scales delay the adjustment of sediment cover relative to changes in sediment flux, thereby modifying sediment-flux-dependent bedrock incision. We further simulate the evacuation of a landslide deposit and examine the resulting bedrock abrasion by the evacuated sediment. For longer sediment transport length scales, sediment accumulates more slowly downstream of the landslide deposit (the cover effect), whereas sediment flux increases rapidly (the tools effect). This mismatch prolongs periods of bedrock exposure under high sediment flux, leading to enhanced bedrock incision. Consequently, variations in sediment transport length scale produce distinct spatial and temporal patterns of bedrock incision during landslide deposit evacuation. Our findings highlight sediment transport length scale as a key control on fluvial responses to episodic sediment inputs in mixed bedrock–alluvial rivers, particularly in tectonically active mountain landscapes where landslides frequently deliver large sediment pulses to river channels.