EVALUATING THE INFLUENCE OF FEEDBACKS BETWEEN EROSION RATE AND WEATHERING ON THE DISTRIBUTION OF ERODIBILITY IN BEDROCK RIVER CHANNELS

Bedrock channel geometry depends on the spatial distribution of rock erodibility within channels. We hypothesize that the interplay between average cross-section erosion rate and weathering processes sets the spatial distribution of erodibility, resulting in a feedback between channel form and process. Specifically, we propose that for any given channel, rocks will vary from relatively unweathered in the thalweg to highly weathered on upper channel banks; and that, between different reaches, measurable rock weathering, and thus erodibility, will negatively correlate with channel erosion rates. We test this hypothesis on three tributaries to the Potomac River underlain by similar bedrock but with varying erosion rates (~0.1 to ~0.8 m/ky). At several cross-sections with different channel geometries, we measured compressive strength with a Schmidt hammer, surface roughness with a contour gage, and density of visible cracks at multiple heights above the thalweg. All measured cross-sections showed significant declines in compressive strength (~10% to ~50%) and increases in crack density (~25% – 45%) with height above the thalweg, while six of nine showed significant increases in surface roughness with height. These data demonstrate increases in the degree of weathering and thus in rock erodibility with height above the thalweg. Furthermore, the difference in measured weathering between the thalweg and the channel banks at each cross-section is inversely related to unit stream power (R² = 0.69 for compressive strength, R² = 0.56 for crack density), providing evidence that the accumulation of weathering effects along bedrock channels is predicated on channel erosion rate. These observations are consistent with laboratory abrasion mill measurements that reveal an increase in erodibility as a function of height above the thalweg, and support model simulations showing that feedbacks between weathering and erosion rate significantly influence channel geometry and gradient.