Constraining the contribution of rock glaciers to the summer hydrology of a high-elevation watershed, Uinta Mountains, Utah, USA

Rock glaciers are common components of mountain landscapes with strong potential to document past and present environmental changes, and a notable vulnerability to future climatic perturbations.  Recent studies have begun to consider the contribution of rock glaciers to high mountain hydrology, with a particular emphasis on the possible role of internal ice as a source of meltwater.  This project utilized automated samplers to collect water discharging from two representative rock glaciers in the Uinta Mountains of Utah, USA.  Additional samplers were deployed at a non-rock glacier spring and along the main stream in this basin.  All samplers ran continuously from the start of July through early October, 2021.  Water from the automated samplers, and from precipitation collectors, was analyzed for stable isotopes with cavity ring-down spectroscopy and hydrochemistry with ICP-MS.  Our findings reveal that water draining from the rock glaciers in mid-summer has a low solute content and notably negative δ¹⁸O, consistent with the melting of lingering snowpack.  As summer progresses, values of δ¹⁸O rise and total dissolved load increases as the influence of this snow-derived water wanes.  In late summer and early autumn, nearly all of the rock glacier discharge can be distinguished from snowmelt, summer precipitation, and groundwater by intermediate values of δ¹⁸O, elevated d-excess, and high abundances of Ca and Mg.  This water is interpreted to come from internal ice that was vulnerable to melting in this warm summer following a snow-poor winter.  The isotopic and hydrochemical fingerprint of this rock glacier discharge can then be used as an end-member, along with groundwater and summer precipitation, for unmixing of the late summer streamwater composition.  This exercise suggests that September discharge in the stream, with a watershed of ~50 km² above the sampling point, contains a detectable component derived from melting internal ice of unknown age within rock glaciers.  An important implication of this conclusion is that late summer/ autumn baseflow in high-elevation streams could decrease in the future as this reservoir of subsurface ice is depleted, particularly in summers following low-snow winters.