VARIABILITY IN FLOW AND COMPOSITION OF GROUND WATER DISCHARGING TO AN ALPINE STREAM

The ground water – surface water interface in alpine environments have received relatively little attention, presumably because alpine streams are perceived to be primarily sourced directly from snow/glacier melt and rain. However recent studies suggest that much of this water may be routed through the ground before discharging into the stream. These studies have indicated that discharge may occur from fractured bedrock, via interflow through thin soils overlying bedrock, and from depositional features such as talus slopes or glacial moraines. The temporal and spatial variability in the flow and composition of discharging ground water has not been well documented, but information on this variability may provide insight into the nature of the complex ground water flow paths in mountain environments and provide instruction on sampling density requirements for alpine hydrology studies. It may also have relevance for local aquatic ecosystems. In this study, a small reach of the upper section of Opabin Creek, in the Lake O'Hara Research Basin in the Canadian Rockies was monitored for two summer seasons. Stream flow measurements along the reach indicate that it experiences a nearly-twenty-fold increase in flow due to ground water discharge at the base of a mixed talus-moraine depositional feature. The chemistry of discharging ground water varied substantially over a 25 m section, with no single location providing a consistently representative sample. These data, along with subsurface tracer tests, suggest a number of different ground water flow paths are contributing, possibly including a buried stream, and that the relative contributions change over the season. There is also evidence of hyporheic exchange, with infiltrated stream water discharging in the middle of this ground water discharge zone during certain times of the year.