A Multimethod Investigation of the Lake Agnes Rock Glacier, Colorado, USA

ABSTRACTRock glaciers are important headwater landforms that play a key role in alpine hydrology. North American rock glaciers are estimated to contain the third largest water volume equivalent globally, and over 3800 rock glaciers have been mapped in Colorado alone. However, these features are often overlooked in alpine water budgets. In this study, we present initial results using geophysical, hydrochemical, and repeat structure from motion data to investigate the ice presence, downslope movement, and influence on stream chemistry of the Lake Agnes rock glacier in the northern Front Range, Colorado, USA. We measured an average velocity of 23 ± 4 cm year−1 in the upper 400 m of the rock glacier, with maximum velocities up to 36 ± 4 cm year−1. Rock glacier–fed streams (n = 2) near the spring remained below 2.5°C throughout the summer, mixed‐source streams (n = 3) remained below 3.5°C, and a non–rock glacier stream (n = 1) reached 13.5°C. Rock glacier streams additionally recorded elevated ion concentrations and pH relative to the mixed‐source and lake–fed streams. Geophysical surveys suggest that the rock glacier has an internal structure consisting of a ~3‐m‐thick active layer, underlain by an ice‐poor layer up to 10 m thick and by an ice‐rich layer up to 18 m thick, with total rock glacier thickness between 20 and 30 m. This study supports the presence of ice within the Lake Agnes rock glacier and documents the rock glacier's influence on basin hydrochemistry. In many basins, including Lake Agnes, the reduced climate sensitivity of rock glaciers and their sustained cold‐water input to mountain streams will likely provide a refuge for cold‐water species in a warming climate.