VARIABLE ERODIBILITY IN BEDROCK-FLOORED CHANNELS PRODUCED BY DIFFERENTIAL WEATHERING

Weathering reduces rock tensile strength. Thus, it is expected that weathering enhances abrasion in bedrock-floored channels, which could influence channel form and gradient and accelerate stream incision. Numerical modeling suggests that differential weathering is a first-order control on channel geometry and slope in some settings. We present field measurements collected at the scale of individual channel cross-sections in several bedrock-floored streams to determine whether rock erodibility is variable across channels. We measured in situ rock compressive strength across the channel perimeter using a Type N Schmidt hammer at 1 metamorphic, 2 granite, 8 sandstone, and 19 limestone channel cross-sections. All 30 cross-sections occur in 9 streams. 17 limestone transects occur within a single stream. We assume compressive strength to be an adequate proxy for erodibility. At each location, we measured rock compressive strength by one of the following methods: 1) sampling at ~10 cm intervals along transects oriented perpendicular to flow or 2) collecting ~50 points along horizontal transects or 0.06 m² grids positioned at vertical intervals of ~0.5 m above the thalweg. We surveyed each cross-section to determine the position of measurement series and precisely describe channel geometry. 2 of 2 granite transects, 4 of 5 sandstone transects, and 12 of 19 limestone transects show a measurable decrease in rock strength between the channel thalweg and channel banks. At 17 transects along a ~2 km limestone channel, 10 of 17 transects show a statistically significant decrease in rock strength between the samples closest to the thalweg and those closest to recent high flow markers. We are currently attempting to identify the factors responsible for generating variable erodibility in some locations but not others.