ARE ROCK GLACIERS WATER SOURCES OR WATER PROCESSORS IN ALPINE STREAM SYSTEMS?

Dickey Peak rock glacier is a bedrock-confined, 0.2 km² rock glacier in the semi-arid Lost River Range, Idaho. Pilot monitoring (2017, 2022) indicated a persistent, very cold stream contrasting strongly with a nearby non-rock glacier stream. Systematic monitoring of June 2023–June 2024 discharge, water temperature and summer electrical conductivity (EC), coupled with stable isotopic analysis, leads to a conceptual model of rock glacier hydrologic behavior as a seasonally evolving shallow aquifer.

Dickey Peak rock glacier stream exhibited persistent discharge and cold temperature through the 2023 season, as did six other rock glacier streams (Stanley and Thackray, this session). That flow continued through the winter months. The stream stabilized at 230 L/s baseflow in mid-June, fluctuating only 10–30 L/s through early June, 2024. Snow cover reached <10% on June 30. Non-rock glacier Cedar Creek (3.6x basin area) declined 550 to 300 L/s during the same period. Dickey Peak stream temperature increased 0.8–2.4°C (mean 1.4°C) and remained constant through winter. Cedar Creek warmed 3.6–7.8°C (mean 6.5°C). Two large late-summer storms (50 mm and 20 mm est. rainfall) strongly affected Dickey Peak stream, with rapid Q increases, and T and EC decreases. Similar early June storms exerted minimal effects on the stream, suggesting water storage at that time. Notably, we observed minimal late-season deuterium excess expected of melting temperate glacier ice and infer that 140% normal 2023 seasonal snowpack dominated water sources.

We interpret the rock glacier stream variations in terms of a seasonally evolving aquifer. We infer early season snowmelt throughflow atop a frozen active layer, with substantial snowmelt and rainfall stored in interstitial pore space and disconnected fractures. The high Q, colder, lower-conductivity late-summer storm outflow indicates a more fully interconnected aquifer, with integrated pore spaces and fractures allowing rapid increase in hydraulic head within the rock glacier and prompting rapid outflow of stored, chilled meltwater. We see little isotopic or EC evidence of 2023 glacier ice melt, and we thus view the rock glacier mass as a seasonally variable snowmelt and rainfall processor, as opposed to a rapidly melting glacier-ice water source. This model suggests rock glacier and outflow stream resilience through near-term alpine warming.