Field measurements confirm that hillslope sediment size varies with elevation and geomorphic process regime at Inyo Creek, California

Abstract:

Correlating the spatial variation of hillslope sediment grain size with geomorphic process regimes is essential for understanding feedbacks between sediment production on hillslopes and channel processes. At our field site at Inyo Creek, California, an elevation gradient in the size of sediment produced on hillslopes has been quantified using cosmogenic nuclides and detrital thermochronometry with samples collected at the outlet. Here we report field measurements of surface sediment size from hillslopes within the catchment, which validate those findings. Specifically, we use multiple field methods to measure hillslope grain size distributions, and correlate size variations with geomorphic process regimes across an elevation gradient. We select sampling sites from maps of predicted grain size created by overlaying landscape attributes in GIS to delineate geomorphic landscape units (GLUs). Geomorphic process regimes include bare bedrock, angle of repose slopes of talus, landslide deposits and soil mantled convex hillslopes. We use tape transects and point counts to quantify size distributions of regolith covered slopes. We also analyze photographs using The Digital Grain Size Project software, and for sediments too small to be resolved in photos we collect bulk samples for sieve analysis in the lab. To measure joint spacing, and infer the initial size distribution of rock fragments produced by bare bedrock, we use combine photographs with measurements made with tape transects, and aerial photographs for inaccessible areas. Our findings indicate that higher elevation slopes do indeed supply coarser sediment. Lower elevations have bimodal size distributions composed of sand with scattered boulders, while at higher elevations, slopes are composed a unimodal distribution of gravel, cobbles, and boulders. While boulder density does not vary significantly with elevation, we find a highly significant linear increase in the fraction of gravel and cobble-sized particles with elevation. Bedrock exposure also increases with elevation, becoming the dominant land cover in the upper catchment; bedrock at lower elevations appears more weathered in comparison to bedrock at higher elevations. Ongoing work includes analysis of joint spacing, bulk samples, and correlation with geomorphic process regimes.