SPATIALLY VARIABLE ROCK ERODIBILITY IN BEDROCK CHANNELS UNDERGOING TRANSIENT RESPONSE TO BASELEVEL CHANGE, GREAT FALLS AREA, VIRGINIA

Field and modeling studies suggest that subaerial bedrock weathering can produce variability in rock resistance within bedrock channel cross-sections. Weathering may weaken rock on the channel banks more effectively than in the thalweg. We hypothesize that channel erosion rate sets the effectiveness of weathering, and therefore the resistance to erosion, across the channel. We present data from tributaries to the Potomac River that are responding to a baselevel drop of ~20 m associated with migration of the Great Falls knickzone. Knickpoint migration on the tributaries has produced variable erosion rates from <0.5 m/ky to ~.8 m/ky, allowing us to assess the variability of rock strength with erosion rate. We used a Type N SilverSchmidt hammer to measure rock compressive strength at multiple heights above the thalweg in channel cross-sections below, within, and above the knickpoints. Measurements were made in or near the thalweg, at recent maximum flow heights, and at several heights in between. At each height, 60 Schmidt Hammer measurements were taken at randomly chosen points. All seven cross-sections studied showed decreases in strength with height. In cross-sections with high erosion rate, moderate erosion rate, and low erosion rate, compressive strength decreased by ~15%, ~45%, and ~55%, respectively. Overall, average rock strength in cross-sections decreased with decreasing erosion rate. We also used a profile gage to measure rock surface roughness. At each height where compressive strength was measured, six roughness profiles were recorded. We used automatic edge detection combined with a moving window analysis to determine an average roughness value at each height above the thalweg. Preliminary results from one channel show that roughness increases significantly with increasing height. This is consistent with predictions that bedrock on the channel margins weathers more effectively than rock in the thalweg.