ABSTRACT:

Rock glaciers discharge meltwater that has undergone extensive water-rock interactions, raising concerns about poorer water quality in alpine areas as rock glacier outflows become a larger part of the water balance with decreasing snowpack. Rock glacier outflows are critical habitat for cold-adapted species, perhaps outweighing negative impacts on water quality. To evaluate geochemical and biological influences of rock glaciers in alpine watersheds, we sampled water chemistry in 80 rock glacier outflows and 23 non-glacial comparison springs across the Rocky Mountains of Utah and Colorado, analyzing for major and trace elements, major anions, stable isotopes, and in situ physical parameters. We also sampled macroinvertebrate communities from six rock glacier outflows and four comparison springs. Comparison of water chemistry with drinking water and freshwater aquatic life criteria identified exceedances for Al, Be, Cd, Cu, Ni, and Zn at some sites. Multivariate analyses revealed no significant chemical differences between rock glacier outflows and comparison springs; variation was instead driven by mountain range and bedrock geology. However, rock glacier outflows exhibited lower temperatures and higher dissolved oxygen concentrations. Macroinvertebrate community composition was more strongly structured by temperature than chemistry, with cold-adapted taxa more common at rock glacier outflows. These findings suggest that a shift toward a greater influence of rock glacier outflows on the alpine water supply is unlikely to degrade downstream water quality, but the loss of cold, stable discharge may result in a lack of thermal refugia for cold-adapted aquatic communities in a warming landscape.