RESEARCH ARTICLE Modelling the effects of climate and flow regulation on ice‐affected backwater staging in a large northern river Prabin Rokaya 1,2 | Daniel L. Peters 3 | Barrie Bonsal 4 | Howard Wheater 1,2 | Karl‐Erich Lindenschmidt 1,2 1 Global Institute for Water Security, University of Saskatchewan, Saskatoon, Saskatchewan, Canada 2 School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan, Canada 3 Environment and Climate Change Canada, Water–Climate Impacts Research Centre, University of Victoria, British Columbia, Canada 4 Watershed Hydrology and Ecology Research Division, Environment and Climate Change Canada, Saskatoon, Saskatchewan, Canada Correspondence Prabin Rokaya, Global Institute for Water Security, University of Saskatchewan, 11 Innovation Boulevard, Saskatoon, SK, S7N 3H5, Canada. Email: prabin.rokaya@usask.ca Abstract In cold region environments, ice‐jam floods (IJFs) pose a severe risk to local commu- nities, economies, and ecosystems. Previous studies have shown that both climate and regulation affect IJF probabilities, but their relative impacts are poorly understood. This study presents a probabilistic modelling framework that couples hydrologic and hydraulic models to assess the relative role of regulated and natural- ized flows on ice‐affected backwater staging. The framework is evaluated at an IJF‐ prone town on the Peace River in western Canada, which has been regulated since 1972. Naturalized flows were generated for the comparison, and ice‐affected backwater profiles were calculated along jams of varying length and location and for different combinations of model parameters and boundary conditions. Results show significant differences in backwater staging (~2 m for a return period of T = 1:10 year) between two study time periods (1973–1992 vs 1993–2012) as compared with two different hydraulic flow conditions (regulated vs naturalized), suggesting a larger role of climate than regulation in backwater staging. However, regulation was found to offset flood risk during the 1973–1992 period and exacer- bate flood risk during the 1993–2012 period. KEYWORDS backwater staging, ice‐jam flood, probability modelling, regulation, risk assessment 1 | INTRODUCTION Ice‐jam floods (IJFs) that occur during river ice breakup have ecological (e.g., restore aquatic conditions) significance in northern rivers and deltas because they are effective in recharging high‐elevation basins (Peters, Caissie, Monk, Rood, & St‐Hilaire, 2016). But they also pose major concern for citizens, authorities, insurance companies, and gov- ernment agencies. The annual financial cost of river ice jams in North America is estimated to be about USD 300 million in 2017 value (French, 2018). Although river ice jamming and IJFs are irregular and complex phenomena, they are primarily governed by channel mor- phology, freeze‐up conditions, ice characteristics, climatic factors, and snowmelt run‐off (Andrishak & Hicks, 2008). Previous studies (e.g., Beltaos, 2003; Beltaos, 2014; Beltaos et al., 2008) have found that among other hydro‐meteorological conditions, fall freeze‐up stage and spring break‐up flows play a significant role in influencing the occurrence of ice jamming and subsequent flooding. In regulated rivers, natural flow regimes are altered by flow control structures (e.g., dams, weirs, and flow diversions) and water extraction. In the case of hydropower reservoirs in cold regions, the release of stored water leads to larger than natural flow conditions during the fall and winter months (when energy demands are higher); whereas natu- rally occurring peak flows during spring and summer months are often dampened by the storage of water (when energy demands are lower). Due to the relatively warmer hypolimnion water coming from reser- voir, ice cover in winter is largely absent immediately downstream of Received: 13 September 2018 Revised: 27 March 2019 Accepted: 28 March 2019 DOI: 10.1002/rra.3436 River Res Applic. 2019;1–14. © 2019 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/rra 1