Rethinking Variability in Bedrock Rivers: Sensitivity of Hillslope Sediment Supply to Precipitation Events Modulates Bedrock Incision During Floods

Bedrock rivers are the pacesetters of landscape evolution in uplifting fluvial landscapes. Water discharge variability and sediment transport are important factors influencing bedrock river processes. However, little work has focused on the sensitivity of hillslope sediment supply to precipitation events and its implications on river evolution in tectonically active landscapes. We model the temporal variability of water discharge and the sensitivity of sediment supply to precipitation events as rivers evolve to equilibrium over 10 6 model years. We explore how coupling sediment supply sensitivity with discharge variability influences rates and timing of river incision across climate regimes. We find that sediment supply sensitivity strongly impacts which water discharge events are the most important in driving river incision and modulates channel morphology. High sediment supply sensitivity focuses sediment delivery into the largest river discharge events, decreasing rates of bedrock incision during floods by orders of magnitude as rivers are inundated with new sediment that buries bedrock. The results show that the use of river incision models in which incision rates increase monotonically with increasing river discharge may not accurately capture bedrock river dynamics in all landscapes, particularly in steep landslide prone landscapes. From our modeling results, we hypothesize the presence of an upper discharge threshold for river incision at which storms transition from being incisional to depositional. Our work illustrates that sediment supply sensitivity must be accounted for to predict river evolution in dynamic landscapes. Our results have important implications for interpreting and predicting climatic and tectonic controls on landscape morphology and evolution. Rivers that carve rock are often used as markers of climate, tectonic, and rock‐type controls in evolving landscapes. To back out these controls from the present form of rivers which erode rock, thorough knowledge of how they evolve and how they respond to changes in climate and tectonics is required. Most models for the evolution of bedrock rivers assume that the rate of erosion always increases with river discharge, as the weight of water on the riverbed increases. Here, we challenge this assumption using a new model for river evolution which accounts for short‐timescale variations in river discharge and supply of sediment from hillslopes to channels. We find that in these systems, the la