Tree ring analysis of hydro-climatic thresholds that trigger ice jams on the Mistassini River, Quebec

This study documents the spatiotemporal variations in the frequency and magnitude of ice jams in the Mistassini River and applies that information to the identification of the hydro-climatic threshold conditions associated with major events. Ice jams cause severe upheavals in water flow, which result in flooding upstream of the ice jam front, and therefore represent a significant geohazard to riparian populations. To analyze the spatiotemporal variations in the magnitude-frequency of ice jams, the Mistassini River was first divided into six different sites representing different geomorphological contexts. A 50-year ice jam chronology was constructed from 85 damaged trees from all of the study sites. This chronology was then coupled with hydro-climatic variables to construct classification trees, which helped identify the conditions and hydro-climatic thresholds favourable to the triggering of ice jams in a predictive model. The results indicate complex interactions between the characteristics of flow, ice cover and river morphology that affect the frequency and magnitude of ice jam events on the Mistassini River. These factors affect the frequency and magnitude of ice jam events. The triggering of extreme events seems particularly influenced by exceptional ice conditions and sites with high sinuosity and islands. Copyright (c) 2015 John Wiley & Sons, Ltd.