Hillslope Sediment Size Distributions Linked to Geomorphic Process Regimes in a Steep Mountain Catchment: Field Data from Inyo Creek, Sierra Nevada, California

Abstract:

The size of sediments produced on hillslopes and supplied to channels regulates river incision and thus landscape evolution in steep mountain catchments, yet the controls on sediment size are poorly understood. Analysis using cosmogenic nuclides and detrital thermochronometry on samples collected at the outlet of Inyo Creek, California, has quantified spatial variation in the size of sediment produced on hillslopes, however field data are needed to validate and explain the findings. We report on a field campaign to measure hillslope grain size distributions, and correlate size variations with topographic, geomorphic, and climatic attributes, across an elevation gradient in this steep catchment. We begin by mapping hillslope geomorphic process regimes, which in this watershed, underlain by granodiorite, are: bare bedrock that erodes by spallation and landsliding, angle of repose slopes of talus, landslide and debris flow deposits, and at lower elevations, soil-mantled convex hillslopes. For each process regime, we select sampling sites to span a wide elevation range. We use tape transects to measure the size of particles >100 mm, the extent of bedrock exposure, and density of vegetation. For finer sediments we collect bulk samples for sieve analysis in the lab. On bare bedrock, we measure joint spacing to infer the size of rock fragments produced. For steep, inaccessible areas we analyze photographs, scaled by objects of known size. Early results suggest that sediment production occurs primarily on bare bedrock surfaces that supply regolith-covered surfaces below, which serve as transport pathways and storage reservoirs. At lower elevations in the catchment, size distributions are bimodal, with only large boulders and fine-gravel and sand. At higher elevations, slopes near the channel have a more continuous distribution, including gravel, cobbles, and small boulders. Results to-date are broadly consistent with the geochemical analysis, which found that higher elevation slopes supply coarser sediment. Our ongoing field measurements of hillslope size distributions at Inyo Creek, combined with additional geochemical data, will help test the hypothesis that sediment size varies systematically with elevation, because of altitudinal gradients in climate and dominant erosional process.