Ice-induced backwater has been shown to be the only method by which ¯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 severity of river ice break-up were analysed. Speci®c emphasis was placed on the roles of ¯ow 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 ¯ow at the time of break-up. Moreover, regulation has actually led to an increase in spring ¯ow originating from the headwater region. Since the mid-1970s, however, spring runo has declined in the downstream portions of the basin unaected by regulation. This has been linked to a decrease in the magnitude of the winter snowpack. Elevated ice levels and winter ¯ows resulting from regulation have further reduced the potential of tributary runo to produce severe break-up ¯oods. Thus the absence of a high-order event between 1974 and 1992 seems to be related to a combined eect of ¯ow regulation and the vagaries of climate.