PREDICTING HILLSLOPE SEDIMENT DISTRIBUTION USING LOCAL CLIMATIC VARIABLES

Landscape evolution and climate are linked by river bed sediments, which regulate the efficiency of river incision into bedrock. The sediment delivered to a stream channel depends on the character of the surrounding hillslopes, which in turn, depends on climatic variables like temperature, precipitation, water residence time and vegetation. Yet far more studies exist on sediment in the stream channel than on the varied processes of sediment production and transport on hillslopes. In particular, there is little in the way of theory or data to predict how climate influences patterns in hillslope sediment size distributions at the watershed scale. To address this knowledge gap, we are investigating hillslope sediment production in the steep granitic catchment of Inyo Creek, in the eastern Sierra Nevada of California. Rock strength does not vary in this catchment so we can focus on the climate-related factors of elevation, slope, aspect, and land cover. These factors vary widely across 2 kilometers of relief, from bare bedrock cliffs at higher elevations to vegetated, regolith-covered slopes at lower elevations. We expect the geomorphic and climatic factors that influence temperature and water residence time, and thus the intensity of chemical versus mechanical weathering, will correlate with resulting hillslope sediment sizes.

We begin by mapping the catchment to categorize geomorphic landscape units (GLUs) by overlaying basic GIS attributes to create polygons of similar inferred sediment production and predict three categories of sediment size on an ordinal scale. The resulting map analysis predicts that 50% of the catchment will be dominated by boulder-size sediment, with smaller sediments dominating lower elevations. From our map of geomorphic landscape units we select field locations within three ordinal size categories and use pebble counts and photogrammetry to quantify hillslope sediment size distributions. Our prediction accuracy is significant and we find a strong positive correlation with elevation. Including slope and aspect with elevation predicts areas where larger clasts dominate even at lower elevations. These results illustrate how climate-controlled hillslope sediment production can influence river sediment supply, and thus bedrock incision and landscape evolution.