PACE OF ROCK FALLS AND CLIFF RETREAT IN YOSEMITE VALLEY, CALIFORNIA, SINCE THE LAST GLACIAL MAXIMUM

Following retreat of the Last Glacial Maximum valley glacier from Yosemite Valley, California, approximately 15,000 years ago, rock falls have become the dominant landscape erosional process. We investigated the postglacial pace of rock falls and cliff retreat from two adjacent granitic cliffs in Yosemite Valley: the southeast faces of El Capitan and Middle Brother. Using airborne lidar data, we estimated talus volumes that have accumulated on the flat valley floor since deglaciation by fitting surfaces to bedrock outcrops and alluvial sediment deposits adjacent to the talus and extrapolating these surfaces beneath the talus. The resulting talus volumes, corrected for porosity, provide a measure of rock fall activity over the past 15,000 years. We also used oblique aerial photographs of El Capitan and Middle Brother taken in 1976 to construct “historical” Structure-from-Motion (SfM) photogrammetry models and compared those to modern (2010-2017) SfM and lidar models, providing a ~40-year rock fall record. Normalizing rock fall volumes by the contributing cliff area and dividing by the time interval provides spatially and temporally averaged cliff retreat rates, a simple metric for comparing postglacial volumes and rates with modern values. The postglacial talus volume of 7.81 ± 2.35 million m³ beneath El Capitan yields a cliff retreat rate of 0.8 mm/yr. The ~40-year rock fall volume for El Capitan of 16,653 ± 1,455 m³ also yields a retreat rate of 0.8 mm/yr. Identical postglacial and modern rates indicate that the rock fall activity observed at El Capitan over the past ~40 years is representative of long-term activity. In contrast, the talus volume of 4.51 ± 1.13 million m³ beneath Middle Brother yields a retreat rate of 0.3 mm/yr, whereas the retreat rate based on the 40-year rock fall volume of 60,133 ± 7,096 m³ is 1.6 mm/yr, more than five times faster than the postglacial rate. This suggests that Middle Brother has experienced a pulse of rock fall activity over the past 40 years that substantially outpaces postglacial rates. Our results also show that adjacent cliffs with broadly similar characteristics (e.g., rock type, steepness, aspect, glacial history) can display markedly different rock fall behavior over a range of timescales, suggesting that subtle geologic differences may be important in controlling rock fall susceptibility.