The hydrology of biological hotspots in a glacierized catchment: developing conceptual understanding of spatiotemporal variation in water sources and flow paths

English Abstract:

Stream emergences on floodplain terraces in glacierized catchments are often widespread and act as important ‘biological hotspots’, typically supporting elevated in-stream biotic diversity, and abundance. However, the water sources and flow paths supporting these stream systems and their spatiotemporal variation remains poorly understood, particularly in the context of a changing climate. It is important to understand how changes in the water balance, given glacial retreat, melting permafrost, and shifting precipitation patterns will affect their structure and function. Here we present the results of a hydrogeomorphic and hydrologic study of one such terrace on the Middle Fork Toklat River, Denali National Park, Alaska (63° 29’ 38.2”N, 149° 58’ 6.0”W), in which a network of 22 nested piezometers and environmental tracers (major ion solutes and stable isotopes) were used to track water sources and establish flow paths. A number of hydrofacies units were identified across the terrace thereby indicating a heterogeneous structure. Groundwater recharge occurred consistently through the spring and summer and the terrace aquifer did not display a flashy response to storm or melt events. We present water chemistry and stable isotope data (δD & δ¹⁸O) which suggest that hillslope runoff processes, and associated water sources (snowmelt, discontinuous permafrost, and summer rainfall), are the dominant control upon terrace aquifer recharge, and not meltwater from up-valley glaciers. Findings suggest colluvial deposits on the slopes above the terrace act as small, but significant temporary storage aquifers, and are an important flow path that supports terrace aquifer recharge. Finally, the heterogeneous structure of the terrace fluvio-glacial deposits and associated spatial variation in water chemistry observed indicate the presence of multiple flow paths in the terrace subsurface and potential significance of preferential flow pathways (PFPs) to the existence of stream emergences. The work highlights the importance of hillslope runoff processes, rather than up-valley glacial retreat, when considering the implications of a changing climate upon these systems.