QUANTIFYING POST-GLACIAL TALUS DEPOSITION IN YOSEMITE VALLEY USING GIS AND SHALLOW SURFACE GEOPHYSICS

Yosemite Valley, California, is frequently affected by slope movement events, the most common of which are rock falls and rock slides. Although a historic database of slope movement events extends back over 150 years, longer-term rock fall rates are needed to place recent events in a geologic context. The single best archive of long-term rock-fall activity is the talus that has accumulated since the last glacier retreated from the valley ~17 ka. We developed a two-part approach to determine talus volumes beneath Glacier Point: 1) ArcGIS analysis of an airborne LiDAR-derived DEM , and 2) shallow surface geophysical investigations. We sampled elevation values from topographic surfaces in the DEM, fit surfaces to these data, and determined the sub-talus morphology by extrapolating these surfaces beneath the talus deposits. Calculated talus volumes range from 1.5 x 10⁶ m³ below the Glacier Point Apron to 3.2 x 10⁶ m³ below the western portion of Glacier Point. To assess the accuracy of the GIS analysis, we used shallow surface geophysical methods to evaluate the depth of the bedrock-talus interface. Preliminary seismic refraction on multiple lines traversing the talus below the Glacier Point Apron indicate a well-defined interface, predicated on seismic velocities, between talus and bedrock. Subsequent analysis with ground penetrating radar (GPR) and resistivity will further constrain talus thicknesses. This geophysical information will allow for an assessment of the GIS techniques, and, ultimately, a more detailed understanding of post-glacial rock fall frequency and magnitude in Yosemite Valley.