Geomorphic Analysis of Boulder Volumes and Surface Roughness Along Talus Slopes in Yosemite Valley, California

Abstract:

Talus slopes in Yosemite Valley, California, are a rich archive of rock fall processes occurring since deglaciation (~ 15 ka). The valley is an ideal natural laboratory for investigating rock fall processes because the cliffs display a wide range of heights, steepnesses, orientations, and granitic lithologies. We measured the spatial distribution of boulder volumes on rock fall-dominated talus slopes along 10 transects at 8 locations in Yosemite Valley. Boulder volumes span 6 orders of magnitude, from 0.003 to 3000 m³. As expected, boulder volumes increase non-linearly downslope, with the largest boulders located at or beyond the base of talus slopes. Boulder volumes are smaller below cliffs composed of more mafic lithologies, likely reflecting the greater fracture density in those cliffs. Moderately tall cliffs (400-550 m) tend to produce larger boulders than the tallest and shortest cliffs. Using airborne lidar data, we calculated talus surface roughness and found modest increases in roughness as a function of downslope distance, likely related to the downslope increase in boulder volume. By quantifying the spatial distribution of boulder volumes, our results can be used to improve future assessments of rockfall hazard adjacent to talus slopes.