River incision into bedrock: Mechanics and relative efficacy of plucking, abrasion, and cavitation

Improved formulation of bedrock erosion laws requires knowledge of the actu al processes operative at the bed. We present qualitative field evidence fr om a wide range of settings that the relative efficacy of the various proce sses of fluvial erosion (e.g., plucking, abrasion, cavitation, solution) is a strong function of substrate lithology, and that joint spacing, fracture s, and bedding planes exert the most direct control, The relative importanc e of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms o n channel bed and banks, and their spatial distributions. We find that pluc king dominates wherever rocks are well jointed on a submeter scale. Hydraul ic wedging of small clasts into cracks, bashing and abrasion by bedload, an d chemical and physical weathering all contribute to the loosening and remo val of joint blocks, In more massive rocks, abrasion by suspended sand appe ars to he rate limiting in the systems studied here. Concentration of erosi on on downstream sides of obstacles and tight coupling between fluid-flow p atterns and fine-scale morphology of erosion forms testify to the importanc e of abrasion by suspended-load, rather than bedload, particles. Mechanical analyses indicate that erosion hy suspended-load abrasion is considerably more nonlinear in shear stress than erosion by plucking, In addition, a new analysis indicates that cavitation is more likely to occur in natural syst ems than previously argued. Cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the flutin g and potholing of massive, unjointed rocks that is otherwise attributed to suspended-load abrasion. Direct field evidence of cavitation erosion is, h owever lacking, In terms of the well-known shear-stress (or stream-power) e rosion law, erosion by plucking is consistent with a slope exponent (n) of similar to 2/3 to 1, whereas erosion by suspended-load abrasion is more con sistent with a slope exponent of similar to 5/3. Given that substrate litho logy appears to dictate the dominant erosion process, this finding has impo rtant implications for long-term landscape evolution and the models used to study it.