Glacier contribution to lowland streamflow: A multi-year, daily geochemical hydrograph separation study in subarctic Alaska

Abstract

Glacier melt water is a critical fresh water contribution of stream discharge for many regions and in high latitudes glacier extent is decreasing with climate warming. However, characteristics of glacierized contributions to subarctic watershed-scale geochemistry and runoff have attracted limited attention. Tracer studies of glacier dominated watersheds are also commonly limited to a few days. We conducted a six-year geochemical hydrograph separation study of melt season daily streamflow in a glacierized watershed in Interior Alaska to estimate glacier, rain, snow, and baseflow contributions. Bulk streamflow water samples were collected daily from the lowlands of Jarvis Creek (634 km2, 3.3% glacier cover) throughout the flow season (late April through September) during 2011–2016. Synoptic sampling of source waters included rain, snow, headwater late-winter streamflow and late-summer glacier terminus discharge. All 1227 source water and bulk streamflow samples were analyzed for stable water isotopes (δ18O and δD) and dissolved ion concentrations (SO42−:Cl− ratio). Bulk streamflow water samples exhibited large seasonal and inter-annual geochemical variation. Considerable inter-annual differences in stable water isotopes within each source water (δ18O values of −15.8 to −20.9‰ for rain, for example) emphasize the importance of informing the mixing model with source waters sampled during the same season as bulk streamflow collection. We estimated glacier discharge to be the largest seasonal contributor to total warm season lowland streamflow (average 35%, ranging from 20% in year 2016 to 44% in 2012) with daily variability from 2 (June 2, 2011) to 80% (September 10, 2013). If the glacier contribution were to cease completely, total warm season bulk stream discharge would be reduced by between 22 and 48% during an above or below average total seasonal rainfall, respectively, with up to an 80% reduction in peak flows. Results from this study suggest that high variability in source contribution of glacierized watersheds, extending across intra- and interannual timescales, challenge any generalization of hydrologic function derived from short-term studies.