THE EFFECTS OF FLOW REGULATION AND CLIMATIC VARIABILITY ON OBSTRUCTED DRAINAGE AND REVERSE FLOW CONTRIBUTION IN A NORTHERN RIVER-LAKE-DELTA COMPLEX, MACKENZIE BASIN HEADWATERS

A distinctive hydrological feature of the Lake Athabasca-Peace-Athabasca Delta (LA-PAD) complex is that flow in channels that drain the system reverses direction when stage on the Peace River exceeds that for the central lakes. This river's hydrology has experienced natural and human induced changes since 1968. This study investigates the importance of spring break-up and open-water induced outflow obstruction and reverse flow contributions to annual lake level maxima under natural (1960-1967), regulated (1976-2004) and naturalized (1976-1996) flow regimes. Obstructed and reverse flow events during spring break-up were common prior to and following flow regulation, suggesting that natural climatic variability in source areas below the W. A. C. Bennett Dam exerted a strong influence on their occurrence. Antecedent hydrological conditions, such as fall freeze-up lake level, break-up magnitude, peak spring flow and initial open-water lake level were significantly associated with annual lake level maxima. During the summer period, lake level was linked to sustained high flows on the Peace River. The river obstructed outflow and contributed reverse flow to the LA-PAD in each year prior to 1968. Following regulation, however, more than half the years did not experience any open-water obstruction and/or reversal, and those that did were characterized by smaller events. The average estimated duration of obstruction was more than two weeks shorter and reverse flow volume was reduced by similar to 90% under a regulated regime compared to a simulated naturalized flow regime. This implied a lowered potential for lateral lake expansion into the delta floodplain in some years. The regulated hydrology could produce large stormflow and high lake levels, but only under extreme climatic events in areas below the dam and/or human-induced alterations to normal reservoir operation. Copyright (C) 2009 Crown in the right of Canada and John Wiley & Sons, Ltd.