Ice jam formation at river confluences: comprehensive field investigation and comparison to laboratory-derived predictive equations

Abstract. Ice jams can cause significant damages to riverine communities during breakup season. Tributaries often play an important role, leading to complex spatial ice processes and occasional flooding. The behaviour of tributaries during breakup as well as their impact on the initiation of ice jams at confluences is very site-specific and depends on ice, morphological and hydrometeorological conditions. The objective of this study is to characterize the interaction between tributary and main river ice during breakup, to quantify hydro-environmental factors controlling breakup sequences and jam formation at confluences, and to evaluate the applicability of laboratory-derived ice jam formation mechanisms to these real-world systems. These objectives were achieved through a comprehensive field monitoring program of four confluences in Québec, Canada (three on the Chaudière River and one on the Sainte-Anne River) over four consecutive winters (2021–2024). Field monitoring included continuous water temperature and depth, time-lapse photography, drone surveys and meteorological data. The results showed that a flow impact mechanism consistently produced ice jams when the flow from ice-free tributary restricted ice passage in the main river. Also, the presence of a downstream ice control structure maintained a stationary ice cover at the confluence, forcing a jam to form on the tributary. Finally, morphological controls including islands and bridge piers at the tributary mouth were found to be preventing ice evacuation at the confluence. These observations were then compared to those developed in the laboratory by Ettema & Muste (2001). It was shown that laboratory derived equations could be used to predict jam formation at confluences when the governing breakup mechanism is well defined and related parameters are adequately measured.