Weathering and abrasion of bedrock streambed topography

Our framework for understanding morphodynamic feedbacks in bedrock rivers is built upon the assumption that rock erodibility is reasonably uniform at the sub-reach scale. Here, we demonstrate that climate-controlled rock weathering combined with bedload abrasion can produce systematic spatial variations in erodibility across bedrock streambed topography. Rock strength data from five channel reaches across the Big Island of Hawai'i show that upstream-oriented rock surfaces are stronger than downstream-oriented surfaces on the same bedrock protrusion. Moreover, the overall strength of these protrusions correlates with local mean annual precipitation rate, demonstrating climatic control of streambed erodibility. Comparing inferred field abrasion rates with experimental flume measurements, we demonstrate that abrasion rates scale exponentially with the orientation of local bed topography relative to streamflow, independent of weathering. However, the spatial variability in abrasion rate across bedrock protrusions is significantly reduced in the field, where large spatial variations in erodibility occur due to weathering. The methods presented here provide a straightforward field-based approach for evaluating the potential influence of weathering on abrasion in bedrock rivers.