EFFECTS OF CLIMATIC VARIABILITY AND FLOW REGULATION ON ICE-JAM FLOODING OF A NORTHERN DELTA

Ice-induced backwater has been shown to be the only method by which fl ooding has supplied water to perched basins within the Peace-Athabasca Delta, one of the world's largest freshwater deltas. The frequency of such events, however, markedly declined in the mid-1970s, To explain this shift, various hydrometeorological conditions that control the se verity of river ice break-up were analysed. Specific emphasis was plac ed on the roles of flow regulation and climate variability. Flow regul ation seems to have produced only minor changes in factors such as ice thickness and strength, and not to have reduced the flow at the time of break-up. Moreover, regulation has actually led to an increase in s pring flow originating from the headwater region. Since the mid-1970s, however, spring runoff has declined in the downstream portions of the basin unaffected by regulation. This has been linked to a decrease in the magnitude of the winter snowpack, Elevated ice levels and winter flows resulting from regulation have further reduced the potential of tributary runoff to produce severe break-up floods. Thus the absence o f a high-order event between 1974 and 1992 seems to be related to a co mbined effect of flow regulation and the vagaries of climate. (C) 1998 John Wiley & Sons, Ltd.