9.28 Bedrock Rivers

Abstract

Bedrock rivers play a critical role in landscape evolution – cutting canyons, creating relief, and driving much of landscape response to changes in climate and tectonics. The bed and banks of bedrock rivers are sporadically rocky but are commonly covered by a thin and fairly continuous layer of alluvium, though bedrock is everywhere close to the surface. Flow hydraulics and channel morphology in bedrock rivers have much in common with coarse-bed alluvial rivers. Interestingly, the width of bedrock channels is similar to alluvial rivers with the same discharge and follows the same scaling with drainage area, suggesting similar controls on channel width despite the difference in substrate strength. Erosion of bedrock in rivers is accomplished by a suite of interacting processes, including abrasion, plucking, cavitation, debris-flow scour, and weathering, and is strongly modulated by river sediment load. Recent models of river incision into bedrock incorporate the dual role of the sediment load as both tools and cover and highlight the importance of discharge variability in the erosion process. Data relating channel steepness, landscape relief, and erosion rate are critical for testing and refining these river incision models, and these increasingly well-established relationships now provide effective tools for diagnosing both spatial and temporal influences of climate and tectonics on landscape evolution. Furthermore, the style and distribution of knickpoints in transient landscapes encodes additional information about climatic and tectonic histories and allows for further discrimination among river incision models that produce similar steady-state forms.