Masters Thesis

Evidence for variations of hillslope sediment size with elevation at Inyo Creek

The size of sediment produced on hillslopes and supplied to channels regulates incision and thus landscape evolution in steep mountain catchments, but controls on sediment size are poorly understood. Geochemical analysis has quantified the spatial variation in the size of sediment produced on hillslopes at Inyo Creek, California. To validate and explain these findings I report a field survey of Inyo Creek that measures the spatial variation in hillslope sediment size. The size distribution of regolith covered slopes is quantified through photo analysis, tape transects, point counts and analysis of bulk samples. The size distribution of bedrock fracture spacing is measured from aerial imagery and field data. Findings indicate that boulder density does not vary significantly with elevation, but there is an increase in the fraction of gravel and cobble-sized particles with elevation and a decrease in the fraction of fine sediment (5 mm) with elevation. All field methods quantifying the size distribution of regolith covered slopes show that median particle size increases with elevation. Findings show bedrock fracture spacing has no trend with elevation. Point count distributions are combined with bulk sample distributions to obtain a set of distributions that are representative of the coarse and fine fraction of sediment on slopes of varying elevation. From these distributions, I fit the Weibull with threshold distribution and use Weibull fit parameters to predict distributions of particle size by elevation across the entire catchment. I fit the predicted distribution of sediment size on the hillslope and the distribution of bedrock fracture spacing into a model of chemical weathering potential. My data supports the hypothesis that because higher elevation slopes are colder, steeper and less vegetated, mechanical weathering processes produce larger sediment while at lower elevation slopes chemical weathering processes produce smaller sediment.

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