<p dir="ltr">Alpine glacier meltwater is an important source of recharge supporting groundwater flow processes in the high mountains. In the face of rapid ice loss, knowledge of response times of mountain aquifers to loss of glacial ice is critical in evaluating the sustainability of alpine water resources for human communities and alpine ecosystems. Glaciers are very sensitive to changes in climate, they advance during periods of global or regional cooling, and they retreat in response to global or regional warming conditions. When the glaciers grow, the equilibrium-line altitude separating the zone of accumulation and zone of ablation on the glacier moves downslope; it moves upslope when they retreat. The latter is not a sustainable condition for the glacier. Previous studies have shown that glacial meltwater is an important source of groundwater recharge. However, we lack fundamental information on the importance of glacial meltwater in mountain groundwater processes such as supporting baseflow generation to alpine streams, perennial flow to alpine springs, and the geochemical evolution of groundwater in mountain aquifers. Thus, continued glacial ice loss may have severe consequences for alpine hydrological and hydrogeological systems.</p><p dir="ltr">Glacier National Park (GNP) and Mount Hood National Forest (MH), both have alpine glaciers. These two study sites show different responses to climate change since their glaciers are in different states of retreat. GNP glaciers are in advanced stages of retreat compared to MH glaciers. Groundwater samples were collected from springs, seasonal snow, glacial ice, and glacial melt (subglacial flow) in GNP and MH. The samples were analyzed for a suite of environmental isotopes and geochemical tracers to address the following questions: 1) How are isotopic fingerprints of glacial meltwater preserved in mountain-block aquifers? What does the isotopic fingerprint of subglacial flow tell us about melting, meltwater processes, and mixing processes? 2) Is the preservation of the isotopic fingerprint of glacial meltwater affected by aspect controls on ice preservation? Aspect is defined as the compass direction of the slope where the glacier is found. 3) What controls groundwater flow and flowpath connectivity from high elevations (near glacier) to lower elevations? What geologic units support groundwater flow to local- and regional-scale springs and flowpath connectivity across spatial scales in each study site?</p><p dir="ltr">The flow of groundwater in mountainous terrain is heavily dependent on the hydraulic properties of the bedrock including presence/absence of dipping layers and structural features, primary and secondary porosity, and presence/absence of ongoing tectonic activity. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) were used to identify the rock units that host groundwater flowpaths and to quantify flowpath connectivity across spatial scales in both study sites. The ⁸⁷Sr/⁸⁶Sr data show that flowpaths in GNP are primarily hosted in the Helena Formation and permeable facies in the Snowslip Formation. Groundwater also flows through alluvium and younger bedrock units, and there is some flow along or through the volcanic sill in the Helena Formation. Hydrostratigraphy also affects groundwater flow and the spatial distribution of alpine springs in GNP. At MH, the rock units hosting flowpaths are young reworked volcanic rock units that are Quaternary in age. Flowpaths in MH appear to be connected across spatial scales since warm springs emerging along the lower southern slopes of Mount Hood preserve stable isotopic signatures of glacial meltwater. In comparison, nearly all the sampled springs in GNP emerge on south-facing slopes. This is not an indication of ice preservation, instead it’s controlled by hydrostratigraphy. In fact, it’s unlikely that high-elevation groundwater is strongly connected to low-elevation sites due to hydrostratigraphy. There are more springs on south-facing slopes at MH as well; however, they do not preserve an isotopic signature of recharge from glacial meltwater except for the warm springs. Springs on north-facing slopes in MH, however, do preserve the signature.</p><p dir="ltr">Tritium (³H) and chlorine-36 (³⁶Cl/Cl) were measured to assess how the isotopic fingerprint of glacial meltwater is preserved in mountain aquifers. The ³H activities in spring water are elevated in GNP and it’s difficult to differentiate between modern precipitation and glacial meltwater. Tritium activities are lower in MH, but it’s also difficult to differentiate between potential endmembers. This discrepancy could imply that glacial meltwater doesn’t contribute to groundwater recharge, but this doesn’t support the Bayesian stable isotope mixing model results of an earlier study. Instead, I infer that englacial mixing processes are affecting the isotopic fingerprint of subglacial melt. An englacial mixing model (EMM) was developed to explain how the isotopic fingerprint of subglacial flow (glacial meltwater) changes in relation to the stage of retreat. The stage of retreat is important because it controls the proportion of glacial meltwater to runoff from snowmelt and rain that enters the englacial network from the surface of the glacier. Mixing occurs in the englacial network, and the mixed water is transported to the base of the glacier. Englacial mixing in conduits, fractures, and moulins affects the ³H and ³⁶Cl/Cl fingerprint of subglacial flow and will, in turn, affect the isotopic fingerprint of recharge from glacial meltwater. For this study, the ³H is not robust by itself; however, ³⁶Cl/Cl shows some additional benefits over ³H. The EMM suggests that the impact of englacial mixing and the influence of modern precipitation on the isotopic composition of subglacial flow increases as the glacier retreats in both GNP and MH. This model is novel to the best of our knowledge. Additional testing of the EMM should be prioritized in the near future.</p>