Process and perturbation in a high elevation alluvial system, Yosemite National Park, USA.

High-elevation alluvial river environments are little studied by geomorphologists despite their sensitivity to disturbance and their critical role as the landform supporting montane meadow ecosystems. Here, we establish the fundamental fluvial functioning and legacy factors inherent to the upper Tuolumne River in Tuolumne Meadows (elevation 2620 m asl), Yosemite National Park, USA. Evidence was drawn from structured field reconnaissance, morphological surveys, and sediment analysis; integrated with LiDAR monitoring and assessments of flood frequency, flood inundation, and sediment transport potential. Results indicate a sinuous single-thread channel in dynamic equilibrium with fluvial processes but with reduced rates of meander activity over the past century. Geomorphic processes are dominated by the influence of snowmelt discharge, partly because atmospheric river events rarely influence this high elevation. Process intensity is thus directly related to snowpack depth with monitored bank erosion rates substantially higher following the deep snowpack of 2016–17. Diurnal cycling of flow means that bedload-transport potential scales very well with total annual discharge volume. The legacy of human activity is unique here, with perturbations caused primarily by early-to-mid twentieth century infrastructure development rather than typical patterns of watershed development. Pre-eminent was 1930s instream aggregate extraction for road improvements. In conjunction with naturally very low rates of sediment supply, the resulting pit is still only one-quarter full and could disconnect downstream sediment supplies for several centuries more. The gravel pit appears to explain incision of the upper channel reach and reduced rates of lateral activity farther downstream. Future impacts will be dominated by climate change, including the potential increased influence of atmospheric river events, reduced snowpack depth, and tradeoffs in sediment production and connectivity as headwater glaciers recede. The upper Tuolumne River and its meadow typify the unusual, but not singular, geomorphology of high-elevation alluvial rivers, whose high ecological and social values justify greater attention. • Rare geomorphic study of a structurally-controlled high elevation alluvial river • Processes regime related to snowmelt-runoff, snowpack depth, low sediment supply • Lateral migration rates reduced in recent decades. • Primary confirmed human legacy is from 1930s instream aggregate mining. • Geomorphic processes and perturbations underpin montane meadow conservation values. [ABSTRACT FROM AUTHOR]