Open-Water and Ice-Jam Flooding of a Northern Delta

Flooding is critical to the ecosystem health of many river-delta environments, particularly to perched-ponds and lakes that are vertically separated from the open-water flow system. This is the case for the Peace-Athabasca Delta in northern Canada, one of the world's largest, freshwater deltas. Unfortunately, this delta has not experienced a major flood since 1974. As a result, significant drying has occurred in the higher-elevation portions of the Delta landscape. This has led to significant changes in, for example, the vegetation regime and the related small-mammal habitat. For almost two decades, popular belief was that drying of the Delta was due to a reduction in the size of open-water floods resulting from upstream flow regulation and/or changes in climate. Following a historically high flood in 1990, however, it was realized that open-water floods were relatively ineffective in flooding the perched bases. A historical analysis of hydrometric records revealed that the major peak-water levels have been produced at the time of break-up. The spring flow driving such events are more related to downstream tributary runoff than flow produced within the headwaters, above the point of regulation. Other ice factors, however, such as thickness, strength, and winter levels, may also have been important in controlling break-up severity.