Hillslope and groundwater contributions to streamflow in a Rocky Mountain watershed underlain by glacial till and fractured sedimentary bedrock

Abstract

Permeable sedimentary bedrock overlain by glacial till leads to large storage capacities and complex subsurface flow pathways in the Canadian Rocky Mountain region. While some inferences on the storage and release of water can be drawn from conceptualizations of runoff generation (e.g., runoff thresholds and hydrologic connectivity) in physically similar watersheds, relatively little research has been conducted in snow-dominated watersheds with multilayered permeable substrates that are characteristic of the Canadian Rocky Mountains. Stream water and source water (rain, snowmelt, soil water, hillslope groundwater, till groundwater, and bedrock groundwater) were sampled in four sub-watersheds (Star West Lower, Star West Upper, Star East Lower, and Star East Upper) in Star Creek, SW Alberta, to characterize the spatial and temporal variation in source water contributions to streamflow in upper and lower reaches of this watershed. Principal component analysis was used to determine the relative dominance and timing of source water contributions to streamflow over the 2014 and 2015 hydrologic seasons. An initial displacement of water stored in the hillslope over winter (reacted water rather than unreacted snowmelt and rainfall) occurred at the onset of snowmelt before stream discharge responded significantly. This was followed by a dilution effect as snowmelt saturated the landscape, recharged groundwater, and connected the hillslopes to the stream. Fall baseflows were dominated by either riparian water or hillslope groundwater in Star West. Conversely, in Star East, the composition of stream water was similar to hillslope water in August but plotted outside the boundary of the measured sources in September and October. The chemical composition of groundwater seeps followed the same temporal trend as stream water, but the consistently cold temperatures of the seeps suggested deep groundwater was likely the source of this late fall streamflow. Temperature and chemical signatures of groundwater seeps also suggest highly complex subsurface flow pathways. The insights gained from this research help improve our understanding of the processes by which water is stored and released from watersheds with multilayered subsurface structures.