RECONSTRUCTING HISTORICAL VOLUME LOSS OF LITTLE ICE AGE GLACIERS IN YOSEMITE NATIONAL PARK

The Lyell and Maclure glaciers in Yosemite National Park are among the longest studied glaciers in North America and are important contributors to streamflow in the upper Tuolumne River, particularly during droughts. The glaciers were up against their Little Ice Age terminal moraines when first mapped and photographed in 1883. Observations from 1903 to 2020 document retreat from these maximum positions, establishing a clear trend of area loss through time. However, accurate predictions for when these glaciers will disappear, and of the resulting impacts to streamflow, require understanding the rate at which ice volume has been lost and how much remains. To calculate past and present ice volumes, we delineated historical glacier extents using historical maps and photographs, satellite imagery, recent (2007-2020) lidar data, and geomorphic features. We then reconstructed glacier surfaces by clipping the previously ice-covered area from lidar data and reinterpolating points within each extent, using the surrounding topography as a boundary condition. We calculated relative volume change by differencing each ice surface against the lidar data. To estimate remaining volumes, we repeated a similar interpolation procedure to reconstruct the subglacial bedrock surface by drawing 2D cross-sections parallel and perpendicular to the ice-flow direction and interpolating those into a 3D surface. We verified our estimated ice thicknesses by drilling to bedrock with a steam drill. Our results indicate that the Lyell Glacier lost 93% of its total volume from 1883 (35.8 million m³) to 2020 (2.5 million m³), with a corresponding decrease in mean thickness from 30 to 9 m. The Maclure Glacier lost 78% of its volume from 1883 (11.1 million m³) to 2020 (2.4 million m³), with a decrease in mean thickness from 29 to 18 m. For both glaciers, 10-14% of the volume loss occurred during drought conditions in 2012-2015, at which time glacier meltwater contributed 75% of the baseflow volumes of the upper Tuolumne River. Predicting when the glaciers will disappear is complicated as they are now sensitive to single-year snowpack accumulations, but regressions indicate they will disappear within 20-40 years. Presently, the glaciers have approximately 4.4 million m³ of water to contribute to streamflow; after the glaciers are gone their critical buffering effect will also disappear.