Ice-jam flood regime of the Peace-Athabasca Delta: Update in light of the 2014 event

Abstract

Major ice jams forming in the lower reaches of Peace River during the spring breakup of the ice cover have been identified as the main agents of flooding and replenishment of the perched basins of Peace–Athabasca Delta, one of the world's largest inland freshwater deltas. The paucity of ice jamming in the lower Peace River following construction of the Bennett Dam (1968) and the potential impacts of climate change have raised concerns regarding habitat degradation. The most recent ice-jam flood, which occurred in 2014, was only the fifth in the post-regulation period and the first since 1997. Despite the sizeable number of years in the regulation period, conventional estimates of flood frequency are shown to be uncertain, but become robust when based on the slope of the cumulative number of floods. The evolution of ice breakup in 2014 was in full accord with previously postulated mechanisms for the flat reach of Peace River, which extends for several hundred kilometres above its mouth. An updated examination of historical hydrometric data at the WSC Peace Point gauge took into account the years 2003–2018, extending the record of the regulation period by 50%. The analysis further confirmed and fine-tuned physically based predictions of the effects of the freezeup level, the breakup flow, the thickness of the winter ice cover, and the accumulated degree-days of thaw on the incidence of ice-jam flooding. Commercially motivated reduction of freezeup flows in recent years likely contributed to the occurrence of the 2014 flood. Total winter snowfall at an index station appears to have reversed a pre-2000 decreasing trend, but its benefit to breakup flow may be moderated by increased incidence of mid-winter thaws.