EFFECT OF LOCAL CLIMATE VARIATIONS WITH ALTITUDE ON HILLSLOPE SEDIMENT SIZE AT INYO CREEK, CA

Quantifying the effect of local climate variations with altitude on sediment produced on a hillslope is essential for understanding how sediment size supplied to a channel regulates river incision and landscape evolution. At Inyo Creek, CA, an elevation gradient in hillslope sediment size was determined using cosmogenic nuclides and detrital thermochronometry of samples collected at the outlet. Local variations in climate and vegetation show a decrease in biomass coverage, a 12^o C decrease in mean annual temperature and an increase in average precipitation by a factor of ~3 between the outlet and the highest elevation of the catchment, Lone Pine Peak. A chemical depletion fraction indicates an altitudinal gradient within the catchment. We report field measurements of the spatial variation in sediment sizes in the catchment. The size distribution of regolith covered slopes is quantified through photo analysis as well as tape transects and point counts measured in the field. For fine sediment, bulk samples are collected for grain weight analysis in the lab. Joint spacing is measured from photographs to infer the initial size distribution of rock fragments produced by bare bedrock. Lower elevations are composed of sand and rounded boulders, while at higher elevations, slopes are composed of angular gravel, cobbles, and boulders. Bedrock exposure increases with elevation and at lower elevations appears more weathered than bedrock at higher elevations. Boulder density does not vary significantly with elevation, but we find a highly significant linear increase in the fraction of gravel and cobble-sized particles with elevation and a highly significant linear decrease in the fraction of sediment less than 5 cm with elevation. For the fraction of scree the median grain weight has a significant linear increase with elevation. The increase in the median grain size with elevation is exponential in the pebble counts. Ongoing work includes analysis of joint spacing, photo samples, and correlation with geomorphic process regimes. We attribute our findings to be a result of the effect of local climate variations. It is hypothesized that because higher elevation slopes are colder, steeper and less vegetated, mechanical processes produce larger sediment while at lower elevation slopes chemical processes produce smaller sediment.