SPATIALLY VARIABLE ROCK ERODIBILITY RELATED TO THE INTERACTION BETWEEN EROSION RATE AND WEATHERING IN BEDROCK-FLOORED CHANNELS, GREAT FALLS AREA, VIRGINIA

Subaerial weathering of bedrock in rock-floored channels can produce variability in rock resistance to erosion within channel cross-sections. Recent results from field studies and numerical modeling suggest that weathering in some settings may preferentially weaken rock in the channel banks relative to the thalweg. We hypothesize that average channel erosion rate determines the relative effectiveness of weathering between the banks and the thalweg. We test this hypothesis on tributaries below the migratory Great Falls knickzone on the Potomac River, Virginia. Erosion rates along tributary profiles vary from significantly below .5 m/ky to approximately .8 m/ky as knickpoints spawned by Great Falls migrate upstream. We used a Type N SilverSchmidt Hammer to measure rock compressive strength at multiple flow heights above the thalweg from sites below, in, and above the migratory knickpoints. At each transect, compressive strength was measured as close to the thalweg as possible, at maximum high flow, and at several heights in between. Approximately 60 Schmidt Hammer measurements were taken at each height, 20 from each of three .06 m² grids aligned parallel to flow at similar elevation. Preliminary results from Difficult Run show statistically significant decreases in compressive strength with elevation at all transects. In the steepest section of the knickzone (e.g., high erosion rate), compressive strength was consistently high and declined by ~12% between the lowest and highest elevation samples. Above the knickzone (e.g., low background erosion rate) compressive strength values were significantly lower and decreased by ~53% with elevation. Below the knickzone (e.g., moderate erosion rate), compressive strength declined by ~49% between the lowest elevation sampled and the elevation of the highest visible high flow marker. This transect showed the greatest absolute decrease in compressive strength with height above the thalweg. The observed dependence of weathering efficiency on erosion rate is consistent with numerical modeling of channel cross-section evolution that incorporates weathering.