HYDROLOGICAL PROCESSES Hydrol. Process. 20, 4009–4029 (2006) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.6417 Ice regime of the lower Peace River and ice-jam flooding of the Peace-Athabasca Delta Spyros Beltaos, 1 * Terry D. Prowse 2 and Tom Carter 3 1 Environment Canada, 867 Lakeshore Road, Burlington, ON, L7R 4A6, Canada 2 Water & Climate Impacts Research Centre, Environment Canada, Department of Geography, University of Victoria, PO Box 3050 STN CSC, Victoria, BC V8W 3P5, Canada 3 Environment Canada at NHRC, 11 Innovation Blvd., Saskatoon, SK S7N 3H5, Canada Abstract: 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. Beginning in the mid-1970s, 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 a habitat for aquatic life in the PAD region. Using archived hydrometric data and in situ observations, the ice regime of the lower Peace is described and quantified, setting the stage for identification of the conditions that lead to ice-jam flooding and replenishment of Delta habitat. The first such condition is the occurrence of a mechanical, as opposed to a thermal, breakup event; second, the river flow should be at least 4000 m 3 /s; and third, an ice jam should form within the last 50 km of the Peace River. The type of breakup event depends on the freeze-up stage and spring flow. The former has increased as a result of flow regulation, and the latter has decreased owing to changing climatic patterns. Both trends tend to inhibit the occurrence of mechanical breakups and contribute to less frequent ice-jam flooding. Potential mitigation strategies are discussed. Copyright  2006 Crown in the right of Canada. Published by John Wiley & Sons, Ltd. KEY WORDS breakup; climate; delta; flood; ice jam; mechanical; regulation; river; thermal Received 4 April 2006; Accepted 9 May 2006 INTRODUCTION The large river deltas of Northern Canada are unique ecosystems, characterized by some of the highest biological productivity and diversity in the world. The Peace-Athabasca Delta (PAD) in northern Alberta (Figure 1) is one of the world’s largest inland freshwater deltas, home to large populations of waterfowl, muskrat, beaver, and free-ranging wood bison. Beginning in the mid-1970s, however, this complex and dynamic region has experienced prolonged dry periods, and considerable reduction in the area covered by lakes and ponds that provide habitat for aquatic life. Previous work has suggested that the combined effects of upstream flow regulation by the W.A.C. Bennett hydroelectric dam (constructed 1968–1971) and climatic variations may have inhibited the formation of extensive spring ice jams that trigger much of the delta inundation (Prowse and Conly, 1998). Ice jams are known to cause much higher water levels than open water floods and are particularly effective in replenishing the higher-elevation, or ‘perched’, basins of the PAD. Since the 1970s, various attempts have been made to restore the damaged ecosystem of the PAD, including the recent use of artificial ice dams to induce ice-jam flooding during the spring breakup of the ice cover (Prowse and Demuth, 1996a). However, the planning for rehabilitation of this important ecosystem has been * Correspondence to: Spyros Beltaos, Environment Canada, 867 Lakeshore Road, Burlington, ON, L7R 4A6, Canada. E-mail: spyros.beltaos@ec.gc.ca Copyright  2006 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.