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 flooding 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 severity of river ice break-up were analysed. Specific emphasis was placed on the roles of flow regulation and climate variability. Flow regulation 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 spring 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 of a high-order event between 1974 and 1992 seems to be related to a combined effect of flow regulation and the vagaries of climate.