ROCK GLACIER HYDROLOGY IN EAST CENTRAL IDAHO

Rock glacier streams in East Central Idaho exhibit consistent hydrologic behavior that differs markedly from snow-only streams. We monitored discharge and temperature of seven rock glacier outlet streams and two non-rock glacier streams through summer and early fall 2023, with ongoing flow and conductivity monitoring of Dickey Peak rock glacier stream since June 2023.

Discharge of rock glacier streams generally reaches baseflow around July 1 as snow cover diminishes to <10%. Baseflow and temperature of <4° C are maintained through early autumn, as snow-fed streams decline and warm continuously. The cold and constant outflows must result from significant ice content in the rock glacier interior. Electrical conductivity rises through the summer season, indicating a component of outflow water that has had longer water-rock contact, possibly from widely hypothesized basal water flow.

Large summer 2023 rainstorms (ca. 10 cm over two days) further inform our understanding. Discharge peaks began ca. 24 hours after rainfall onset, with all rock glacier streams displaying sharp discharge peaks associated with drops in temperature and conductivity. We infer that while sub-rock glacier water flow may increase through summer months, the storm discharge is dominated by fresh water stored in contact with ice.

We propose a model for rock glacier outflow akin to a seasonally evolving aquifer, while lacking direct knowledge of internal structure. During the snowmelt period, large rainstorms produce rapid but muted stream responses. We infer that rain and meltwater flows in coarse talus on top of the seasonally frozen active layer and feeds quickly into the stream. Through the summer, cold and consistent water flow is maintained by gradually increased intergranular and fracture flow within the evolving perennial ice body, coupled with higher-EC basal flow through inferred sub-ice sediment . Late summer storm outflows of fresher, colder water suggest that rain water is able to infiltrate the perennial ice body, in which snowmelt and ice melt have been gradually stored in evolving intergranular spaces and fractures. Rapid infiltration of rain water raises hydraulic head in the rock glacier interior and drives rapid outflow of cold, low-conductivity water.