TESTING THE GROUNDWATER RIDGING HYPOTHESIS OF STREAMFLOW GENERATION DURING SNOWMELT IN A FORESTED CATCHMENT

Hydrometric and isotopic techniques were used to test the groundwater ridging hypothesis of streamflow generation during snowmelt in a forested catchment on the Canadian Shield. The catchment contains a central wetland that drains to an ephemeral channel. Hydrograph separation using H-2 as a tracer indicated that approximately 60% of total stream flow during snowmelt was supplied by pre-event water. This was supported by hydrometric results. However, contrary to the groundwater ridging hypothesis, the response of water-table levels in near-stream areas to initial melt inputs failed to promote a rapid flux of ground water to the wetland and stream. All stream flow originated as saturation overland flow from the wetland, and groundwater flow during much of the melt was directed down the catchment's main axis and back under hillslopes surrounding the wetland and channel. Although the hydrograph separation indicated pronounced short-term variations in pre-event discharge from the catchment, groundwater fluxes to the wetland remained relatively invariant following initiation of surface saturation. Streamflow fluctuations appear to have been generated by displacement of water held in surface storage, induced by melt and rainwater inputs to the wetland. This stored water had an isotopic signature intermediate between that of event and pre-event water. Suggested oscillations in pre-event runoff were an artifact of the mixing model approach to hydrograph separation, rather than the result of rapid changes in groundwater fluxes to the wetland.