WINTER STREAMFLOW VARIABILITY IN 2 GROUNDWATER-FED SUB-ARCTIC RIVERS,YUKON-TERRITORY, CANADA

Frequent discharge measurements were made during the winter of 1994-19 95 in two groundwater-fed streams near Whitehorse, Yukon Territory, to evaluate the nature and causes of winter discharge variability in sub -Arctic rivers. Observations were also made of near-stream hydraulic h ead, snowmelt percolation, and water quality. A linear reservoir model provided a poor fit to the streamflow recessions at both rivers, wher eas three relatively complex models provided good fits to the data use d for calibration. A pronounced discharge depression occurred at M'Cli ntock River associated with an increase in stage at freeze-up. The vol ume of water represented by the depression was about three times the m aximum amount that could be accounted for by channel storage. This dis crepancy could have been caused by stream-aquifer interactions. Piezom etric observations were consistent with a reversal of hydraulic gradie nt across the stream bed, which would block or reduce groundwater infl ow, as well as cause water to go into bank storage. A sustained discha rge depression did not occur at Ibex River, probably because the volum e of channel storage is small with respect to discharge and could be s atisfied over the period of freeze-up without measurable deviation fro m the recession trend, and because the near-stream hydraulic gradients were strong enough not to be influenced by stage increases. Discharge at Ibex River was uncorrelated with air temperature. At M'Clintock Ri ver, residuals from the layered linear reservoir model appeared to be weakly correlated with air temperature.