Tree rings as indicators of ice jam regimes, triggering conditions and geomorphological impacts : the case of the Mistassini River (Eastern Canada)

English Abstract:

Tree ring records provide invaluable sources of information concerning river ice jam dynamics. They can help researchers rely on a robust spatio-temporal framework that facilitates the analysis of triggering conditions, frequency-magnitude relationships, and geomorphological impacts associated with such extreme events. Here, we provide field evidences that dendrochronological markers have contributed to shed light on river ice jam dynamics in the Mistassini River watershed (Eastern Canada). First, an exhaustive sampling of ice scars along the banks of this highly problematic river revealed that hydro-climatic conditions prevailing during both freeze-up and break-up periods have a tremendous influence on the triggering of river ice jams in the watershed. Based on such relationships with climate, a predictive classification model was constructed and accurately predicted 88% of the events that occurred in the Mistassini River since AD 1960. Second, dendrochronological surveys were coupled to an extensive geomorphological description of river banks. This allowed linking the occurrence and distribution of ice-induced landforms to the properties of the ice jam regime (mostly downstream changes in frequency – magnitude). Among other things, our analysis revealed that extensive ice-related erosion did not necessarily occur where ice jams were the most frequent, but is maintained where events had the greatest magnitude. Our works revealed that tree ring markers may provide a strong basis that can orient risk management strategies, particularly in context where observations of past ice jams are scattered, if not inexistent.