USING IN SITU MEASUREMENTS TO EVALUATE THE VARIABILITY OF ROCK WEATHERING ACROSS BEDROCK-FLOORED CHANNELS

Field and modeling studies suggest that bedrock channels equilibrate to baselevel change through geometry and slope adjustment to imposed discharge, sediment supply, and substrate erodibility conditions. Here we explore the hypothesis that differential weathering may lead to differences in erodibility across bedrock-floored channels. We hypothesize that cross-channel variation in erodibility is an important but largely ignored control on channel geometry. We present in situ measurements of rock compressive strength obtained with a Schmidt hammer in 31 channel cross-sections. Cross-sections were located in sedimentary and meta-sedimentary rock types in arid and humid regions. Weathering has been shown to reduce rock compressive strength, and hence this data provides a semi-quantitative measure of the extent of rock weathering. Preliminary analysis suggests that Schmidt hammer values decrease as height above the thalweg increases in many channels. Relative to thalweg values, average values decreased from 5-30% at 0.5 – 1 m and 10-45% at 1 – 1.5 m above the channel thalweg. And when transects were more than 1.5m above the channel thalweg, the average compressive strength decreased 40 – 60%. We attribute the decrease in Schmidt hammer values with height to increases in porosity, alteration of minerals, and weakening of cementation by weathering. We attribute deviations from this trend to differences in lithology and/or channel geometry. We are currently completing geochemical analyses of rock cores collected at multiple heights above the thalweg to determine changes in weathering indices with height in the channel.