Determining the Controls on Bedrock Exposure on Steep Hillslopes in the Southeastern Sierra Nevada and Inyo Mountains, California

Steep landscapes often contain a mix of soil-mantled and bare bedrock hillslopes. The transition between soil mantled and bedrock hillslopes depends on the balance between soil production and erosion rates, and can be influenced by factors including tectonics, climate and vegetation, lithology, and disturbances such as wildfires. Untangling the relative impact of these factors is important for understanding how mountainous landscapes evolve and may respond to long-term changes in the tectonic and climatic environment. While climate and lithology are often mentioned as controls on rates of erosion and soil production, it is unclear how these factors influence the development of steep landscapes where bedrock is exposed in patches at the surface. To resolve the potential roles of climate and lithology in these landscapes, we are studying the variability in morphology and bedrock exposure in two locations: the southeastern Sierra Nevada and the Inyo Mountains, California. The study sites consist of multiple small headwater catchments underlain by granitic rock that span a range of climates, tectonic forcings, and bedrock compositions. We aim to use these sites to reveal how bedrock exposure and erosion rates vary across these gradients to determine the major controls on the long-term evolution of these catchments. Using a combination of lidar analysis and field observations, we have mapped the bedrock exposure in each site, measured soil thickness and bedrock fracturing, and collected soil and bedrock samples to determine their chemical compositions and degree of weathering within the Sierra Nevada site. In both study sites local slope angles serve as a reliable proxy for bedrock exposure with slopes over 42 degrees indicating bedrock. At the Inyo Mountain site the pattern of bedrock exposure corresponds to a region of rapid channel incision downstream of a knickpoint. Preliminary observations along a gradient of climate and relief in the southeastern Sierra Nevada do not reveal straightforward patterns in soil depth and bedrock exposure across hillslopes, suggesting a more nuanced interplay between bedrock composition, fracturing, and vegetation that we are currently investigating.