Rock glaciers and related periglacial landforms in the Sierra Nevada, CA, USA; inventory, distribution and climatic relationships

Abstract

Rock glaciers and related periglacial rock-ice features (RIFs) are abundant yet overlooked landforms in the Sierra Nevada, California, where they occur in diverse forms. We mapped 421 RIFs from field surveys, and grouped these into six classes based on morphology and location. These categories comprise a greater range of frozen-ground features than are commonly described in rock-glacier surveys. Mapped features extended from 2225 to 3932 m (modern, mean 3333 m), occurred mostly on NNW to NNE aspects, and ranged in apparent age from modern to relict (late Pleistocene). Many of the smaller features mapped here are not readily discernible with remote (e.g., air photo) observation; field surveys remain the best approach for their detection. We interpreted the presence of outlet springs, basal lakes, suspended silt in outlet streams, and fringing phreatophytic vegetation, in addition to morphologic indications of current rock movement, as evidence for interstitial ice, either persistent or seasonal. The six classes were distinct in their geographic settings and morphologic conditions, indicating process-level differences. To assess modern climate, we intersected mapped locations with the 30 arcsec PRISM climate model. Discriminant analysis indicated significant differences among the climate means of the classes with the first three canonical vectors describing 94% of the differences among classes. Mean annual air temperatures (MAAT) for modern features ranged from 0.3 to 2.2 °C; mean precipitation ranged from 1346 to 1513 mm. We calculated differences between modern and Pleistocene temperatures in two ways, one based on elevation differences of modern and relict RIFs (662 m) and standard lapse rate, the other using PRISM estimates. For the first, we estimate the difference in MAAT as −3.9 °C (range −2.2–to 7.9 °C); from PRISM, the difference was −3.3 °C (range −1.0 to –6.1 °C). In that persistent snowfields and glaciers are retreating in the Sierra Nevada under warming climates, RIFs will likely become increasingly important in prolonging water storage during the warm season and providing small but distributed water reserves for biodiversity and runoff. Their presence and water contributions would benefit by further hydrologic study.