LATEST PLEISTOCENE AND HOLOCENE LANDSCAPE EVOLUTION OF YOSEMITE VALLEY, CALIFORNIA

Landforms and sediments of the iconic Yosemite Valley, in the glacially-modified Sierra Nevada of Central California, record evolution of the Merced River catchment since the ~15 ka retreat of the Last Glacial Maximum (LGM) valley glacier. Evaluating the development of the Merced River, and its likely response to continued human activity and future climate change, requires accurate depiction of landscape adjustments driven by sedimentation and erosion. We combine surficial geologic mapping of Yosemite Valley with stratigraphic investigation, geomorphic analyses, and geochronology to test the sensitivity of sediment transport over various timescales. The high volume of valley-margin deposits in comparison with valley-bottom material demonstrates the importance of glacial debuttressing and valley-wall erosion to the postglacial sediment supply. Alluvial fan deposits and talus flanking the valley walls interfinger with valley-bottom stratigraphy comprised of glacial till, lacustrine silts, and fluvial sediments. Recessional moraines, episodically emplaced rock avalanches, and alluvial fans impeded flow of the Merced River and its major valley-bottom tributary, Tenaya Creek, each grading to different local base levels. This predominantly aggradational regime shifted to incision by the mid Holocene, likely due to diminishing supply of glacial sediment, leaving expansive late Holocene terrace surfaces inset up to ~9 m. ¹⁴C charcoal ages from terrace fill and cosmogenic ¹⁰Be exposure ages from rock avalanche boulders reveal an apparent gap in the record (~4.7–9.3 ka), possibly indicative of valley-wide meandering. Pending luminescence ages will further clarify the timing of integration for Tenaya Creek and the Merced River and history of fill underlying valley-bottom meadows. Widespread cut and fill sequences demonstrate subsequent river incision and reoccupation as early as ~500, 800, and 1600 cal yr BP, generally consistent with regional climate forcing. Historical modifications to local base level at the El Capitan LGM moraine and widespread removal of log jams further lowered the channel bed over the past two centuries. These findings, including the record of land-use disturbances, help inform ongoing river-restoration work and illuminate the geologic evolution of this celebrated landscape.