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Rock glaciers, characterized by a rocky active layer insulating a deforming, ice-rich core, are an often-overlooked component of the cryosphere in alpine watersheds. Ablation of the underlying icy core, although low, can provide cold meltwater to mountain headwater environments in the driest months of the year and support sensitive cold-adapted aquatic organisms after spring snowmelt runoff recedes. It is unknown what portion of spring and summer discharge is diverted laterally by the permeability barrier of a rock glacier, or what portion of late summer baseflow is provided by groundwater from rock glacier basal melt. At Imogene Rock Glacier, a 0.15 km2 rock glacier at 3450-3750 m in the San Juan Mountains of Colorado, I gauged outlet streamflow, measured water chemistry, and dated water to document seasonal variations.Ìý

I compare the rock glacier outlet with groundwater seeping from a mining adit and with an adjacent alpine stream. The rock glacier outlet water temperature was consistently colder (average 1.5±1.0°C) and had smaller diurnal variations than the alpine stream, which averaged 5.6±3.1°C. The outlet remained flowing over a month later into the summer that the nearby stream, decreasing from 59 L/s in June to when flow stopped in August, a decline that scales roughly with the loss of snow-covered area. I found that the age of outlet water from dissolved inorganic carbon increases through the summer, suggesting a decline in snowmelt and supraglacial flow path contributions, until a September heavy rain event revived them. The 4,150±49 BP age of rock glacier discharge in August 2025 is consistent with Holocene aged ice (Lehmann et al., 2025) or groundwater contributing to baseflow. Adit discharge decreased in age through summer, suggesting that a deeper groundwater reservoir mixes with a shallower (snowmelt fed?) component that must pass through the vadose zone. Hydrochemistry and isotopic signatures of melt water and other environmental waters show that rock glacier runoff is measurably distinct compared to snowmelt, precipitation, and groundwater source. The seasonality of the isotopic values and the anticorrelated trends in age between the rock glacier outlet and the groundwater adit, lead me to believe that rock glaciers route water through distinctive flow paths, such as laterally around an impervious icy core, and therefore have a unique influence on the hydrology of small alpine catchments.