Flooding is critical to the ecosystem health of many river-delta envir
onments, particularly to perched-ponds and lakes that are vertically s
eparated from the open-water flow system. This is the case for the Pea
ce-Athabasca Delta in northern Canada, one of the world's largest, fre
shwater 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 sign
ificant 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 clima
te. Following a historically high flood in 1990, however, it was reali
zed 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 bre
ak-up. The spring flow driving such events are more related to downstr
eam tributary runoff than flow produced within the headwaters, above t
he point of regulation. Other ice factors, however, such as thickness,
strength, and winter levels, may also have been important in controll
ing break-up severity.