Variability of erodibility in rock-floored channels produced by differential weathering

The erosion of bedrock-floored channels is a critical process governing the rate of landscape evolution in many settings. Field and modeling studies suggest that bedrock channels evolve toward equilibrium through adjustment of geometry and slope to the imposed discharge, sediment supply, and substrate erodibility conditions. Recent numerical modeling of rock-floored channel cross-sections suggests that equilibrium channel geometry and slope are sensitive to variation in rock erodibility along the channel perimeter. However, few field studies have focused on systematic measurement of rock erodibility across rock-floored channels. We hypothesize variations in weathering intensity and duration across some channels results in variable erodibility. To determine if erodibility varies in some channels, we used a Type N Schmidt hammer to measure in situ compressive strength in channels floored by sandstone (3 sites, Utah), basalt (2 sites, Hawaii), granite (3 sites, Virginia) and limestone (2 sites, Virginia). Rock compressive strength is assumed to be an adequate proxy for erodibility (Sklar et al., 2001). At each site, we collected compressive strength measurements along transects oriented perpendicular to flow direction. We surveyed the transects to determine the position of measurements within cross-sections and to estimate width/depth ratios. Of 19 transects measured in sandstone channels, 10 show significant decreases in compressive strength (up to 60%) with increasing height above the channel thalweg. Five sandstone transects collected from a rapidly eroding slot canyon show no significant change in compressive strength with height. Transects on basalt and 2 of 3 transects on granite show significant compressive strength decreases with increasing height. Transects on limestone, however, reveal compressive strength increases with height above the thalweg. The width-depth ratios in the granite channels are ~10 times greater than the limestone channels, despite similar hydroclimatic conditions and erosion rates, consistent with the hypothesis that variation in erodibility across channels is an important control on geometry. We will present geochemical analyses from microprobe-derived element maps on thin sections cut from cores collected from multiple locations within the cross-sections discussed above. Initial geochemical analyses from transect in sandstone suggests weathering increases with height above the thalweg. This observation is consistent with our hypothesis that differential weathering is responsible for variability in compressive strength within the cross-section.