ESTIMATING HOLOCENE EROSION RATES FROM THE ALLUVIAL FANS OF SMALL CATCHMENTS USINGRADIOCARBON DATING AND LIDAR TO ASSESS VALLEY ASYMMETRY DEVELOPMENT

Alluvial fans can be used to estimate source-catchment erosion rates if the majority of the erosion is recorded in alluvial fans, and the timing and volume of deposition can be established. We estimate Holocene erosion rates from alluvial fans produced by steep, small, ephemeral headwater catchments, where episodic accelerated erosional events (e.g. debris flows and sheetfloods) are the primary mechanisms that transport sediment out of catchments and deposit alluvial fans at outlets. Catchment-averaged sediment yields (i.e. denudation rates) are constrained by 1) determining deposit ages by radiocarbon dating, 2) reconstructing alluvial surfaces from surface remnants evident in LiDAR-derived elevation data, 3) extrapolating age/depth relationships across fan surfaces, and 4) normalizing volumetric rates of deposition by source catchment area. Alluvial fan are predominantly composed of fire-related debris flow and sheetflood deposits. Catchment-averaged sediment yields for individual events are expectedly lower than for larger, more rapidly incising catchments in the region, but are comparable to local uplift and incision rates. Terrain aspect is a major source of variability for coupled eco-hydro-pedo-geomorphic processes in the region, culminating in valley asymmetry, and altering the type of dominant erosional processes (e.g. debris flows vs. sheetfloods). However, aspect does not appear to have a significant effect of Holocene catchment-averaged erosion rates, suggesting that the balance between the magnitude and frequency of erosional events result in similar time-averaged rates. Changes in erosional processes, and the land surface gradients associated with valley asymmetry development, appear to counteract aspect-induced differences in forces driving and resisting erosion, and cause valleys to achieve a dynamic equilibrium where land surface denudation rates are equal on opposite valley sides (i.e. steady state valley asymmetry).