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

<p>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.&#160; 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.&#160; This project utilized automated samplers to collect water discharging from two representative rock glaciers in the Uinta Mountains of Utah, USA.&#160; Additional samplers were deployed at a non-rock glacier spring and along the main stream in this basin.&#160; All samplers ran continuously from the start of July through early October, 2021.&#160; Water from the automated samplers, and from precipitation collectors, was analyzed for stable isotopes with cavity ring-down spectroscopy and hydrochemistry with ICP-MS.&#160; Our findings reveal that water draining from the rock glaciers in mid-summer has a low solute content and notably negative &#948;¹⁸O, consistent with the melting of lingering snowpack.&#160; As summer progresses, values of &#948;¹⁸O rise and total dissolved load increases as the influence of this snow-derived water wanes.&#160; 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 &#948;¹⁸O, elevated <em>d</em>-excess, and high abundances of Ca and Mg.&#160; This water is interpreted to come from internal ice that was vulnerable to melting in this warm summer following a snow-poor winter.&#160; 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.&#160; 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.&#160; 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.</p>