No abstract

Abstract. Understanding how a bedrock river erodes its banks laterally is a frontier in geomorphology. Theories for the vertical incision of bedrock channels are widely implemented in the current generation of landscape evolution models. However, in general existing models do not seek to implement the lateral migration of bedrock channel walls. This is problematic, as modeling geomorphic processes such as terrace formation and hillslopechannel coupling depends on the accurate simulation of valley widening. We have developed and implemented a theory for the lateral migration of bedrock channel walls in a catchment-scale landscape evolution model. Two model formulations are presented, one representing the slow process of widening a bedrock canyon and the other representing undercutting, slumping, and rapid downstream sediment transport that occurs in softer bedrock. Model experiments were run with a range of values for bedrock erodibility and tendency towards transport-or detachment-limited behavior and varying magnitudes of sediment flux and water discharge in order to determine the role that each plays in the development of wide bedrock valleys. The results show that this simple, physicsbased theory for the lateral erosion of bedrock channels produces bedrock valleys that are many times wider than the grid discretization scale. This theory for the lateral erosion of bedrock channel walls and the numerical implementation of the theory in a catchment-scale landscape evolution model is a significant first step towards understanding the factors that control the rates and spatial extent of wide bedrock valleys.

Through the delivery of water in snowmelt, climate should govern the rate and extent of saprolite formation in snow-dominated mountain watersheds, yet the mechanisms by which water flows deeply into regolith are largely unexplored. In this study we link rainfall, snow depth, and water content data from both soil and shallow saprolite to document vadose zone dynamics in two montane catchments over 2 years. Measurements of snow pack thickness and soil moisture reveal strong contrasts between north-and south-facing slopes in both the timing of meltwater delivery and the duration of significant soil wetting in the shallow vadose zone. Despite similar magnitudes of snowmelt recharge, north-facing slopes have higher sustained soil moisture compared to south-facing slopes. To help interpret these observations, we use a 2D numerical model of vadose zone dynamics to calculate the expected space-time moisture patterns on an idealized hillslope under two wetting scenarios: a single sustained recharge pulse versus a set of short pulses. The model predicts that the duration of the recharge event exerts a stronger control on the depth and residence time of water in the upper unsaturated zone than the magnitude of the recharge event. Model calculations also imply that water should move more slowly through the subsurface and downward water flux should be substantially reduced when water is applied in several pulses rather than in one sustained event. The results suggest that thicker soil and more deeply weathered rock on north-facing slopes may reflect greater water supply to the deep subsurface.

Wide, fluvially carved bedrock valleys are found around the world, but little is known about the processes and mechanisms controlling their formation. We know that rates of valley widening are controlled by stream discharge and bedrock lithology, with larger streams and softer rocks resulting in wider valleys. But lithology also controls widening mechanisms that cause valleys with contrasting bedrock strength to widen in fundamentally different ways. The effect of distinct widening mechanisms on bedrock valley width remains unquantified. We analyze a new topographic data set from various lithologies that demonstrates that bedrock valleys in soft lithologies widen downstream twice as fast as valleys in intermediate lithologies. These downstream valley‐widening rates also emerge from landscape evolution model simulations using two end‐member valley‐widening mechanisms. The topographic data show that lithology and mechanism of valley widening are linked and that different widening mechanisms can control bedrock valley development along river courses.