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Abstract Major ice jams forming in the lower reaches of Peace River during the spring breakup of the ice cover are the main agents of flooding and replenishment of the Peace–Athabasca Delta ecosystem. Their paucity 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 the fifth in the post-regulation period. Relative to previous occasions, the 2014 event was monitored in considerable detail. Unusually complete water level recordings at Delta hydrometric stations, high-water-mark elevations, and aerial observations furnish adequate quantitative evidence for application of numerical models. Ice-jam modelling capability is essential to quantification of flow magnitudes that help replenish the Delta and can facilitate assessment of climate-change impacts on flood frequency and development of adaptation strategies. Following a brief chronology of the 2014 ice breakup in the lower Peace River and presentation of key hydrometric data, the user-friendly, public-domain model HEC-RAS is briefly described and its setup for the lower Peace River outlined. It is then calibrated and validated on two ice jam configurations that occurred on distinct dates. Application of the calibrated model to different ice-jam scenarios indicated that jams of moderate, rather than extreme, length are more effective in flooding of the Delta. Highlights • Peace River ice jams cause ecologically vital flooding of Peace-Athabasca Delta • Ice-jam flood of 2014 only fifth since regulation in 1968 and first since 1997 • Relatively well documented event; adequate data for numerical modelling • HEC-RAS model calibrated and validated on jam configurations of May 3 and 7 • Model used to determine optimal jam configuration for widespread flooding [ABSTRACT FROM AUTHOR]

The Peace-Athabasca Delta (PAD) in northern Alberta is one of the world's largest inland freshwater deltas, home to large populations of waterfowl, muskrat, beaver, and free-ranging wood bison. Following construction of the Bennett Dam in the late 1960s, a paucity of ice-jam flooding in the lower Peace River has resulted in prolonged dry periods and considerable reduction in the area covered by lakes and ponds that provide habitat for aquatic life in the PAD region. Past studies have shown that both regulation and climate have contributed to the drying trend, via increased freezeup levels and reduced breakup flows, respectively. However, it has not so far been possible to assess the relative impacts of these two factors. This question is addressed herein by developing quantitative relationships based on the concept of conditional probability. The new methodology is first tested against observed frequencies of ice-jam floods both before and after regulation; it is subsequently applied to quantify the effects of climate and of regulation on the post-regulation reduction in ice-jam flood frequency. The results indicate that both factors have contributed significantly to the drying of the PAD, with regulation having had the more pronounced effect. Complications that may arise from secondary effects of regulation on breakup flows or of climate on freezeup levels are discussed and quantified. It is shown that such effects do not alter the overall conclusions of the paper. [ABSTRACT FROM AUTHOR]