Influence of Autogenic Bedrock Steps on Long Profile Development

Abstract:

Abrupt changes in the slopes of bedrock rivers, or knickpoints, are often attributed to external perturbations in river base level or changes in rock type. However, knickpoints have also been proposed to form as a result of internal system dynamics. This may occur due to feedbacks between flow dynamics, bedrock erosion, and sediment transport that allow the formation of bedrock steps, such that differences in erosion rates between bedrock steps and their neighboring reaches may allow the development of knickzones. The impacts of autogenic bedrock step formation on river longitudinal profile evolution have yet to be explored in detail, particularly whether their presence can create knickzones with 100 m to 1000 m of relief similar to those generally attributed to heterogeneous lithology or external forcing. Here we modify a detachment-limited stream power model to explore the influence of bedrock step formation on river long profile form. We do this by introducing a threshold slope above which erosion rate changes to incorporate potential increases or decreases in erosional efficiency resulting from the formation of bedrock steps. Our modeling results suggests there are diagnostic features, such as step-changes in river gradient once channels have exceeded the threshold slope value, reaches with constant gradient above the threshold slope, and characteristic increases or decreases in relief, that can be used to identify river profiles that experience bedrock step erosion. These predictions are consistent with many features of longitudinal profiles in waterfall-dominated reaches in the Sierra Nevada, California. These results show promise to help differentiate knickpoints related to perturbations in external forcing (as are commonly used to infer geologic history using landscape form), from those that are generated by internal system dynamics.