The spatial variability of ice-jam bank morphologies along the Mistassini River (Quebec, Canada): an indicator of the ice-jam regime?

Abstract

This study aims to investigate ice-jam regimes on the Mistassini River by analyzing the impacts of ice-jams on riverbank morphologies and ecosystems. The Mistassini River experienced severe ice-jams in May 2011, which caused the destruction of 20 riverside houses. Geomorphological and ecological descriptions of riverbanks helped identify and spatialize ice impacts along four sections of the river with contrasted morphologies: meanders, linear with islands, linear, and sinuous. Channel characteristics were investigated to document the typical spatial context in which ice-impact features dominate on the Mistassini River. In each section, ice-jam frequencies and magnitude were determined through the dendrochronological analysis of ice scars found on riparian trees. Our results show that ice-impacted banks are characterized by a two-level structure separated by a steep (average slope = 25 %) and irregular talus overlaid by freshly deposited alluvia. Those banks are colonized by abundant shrubs and few trees, both presenting numerous ice scars. This bank type forms during severe eroding events where the ice flood reaches levels that are much higher than bankfull. Most ice-impacted banks tend to occur in two preferential hydro-geomorphological contexts: (1) a downstream increase in the flow area and width–depth ratio within a mild slope channel; (2) a sudden narrowing in an entrenched channel. We finally show that the proportion of two-level banks increases in the downstream direction as ice-jam frequencies decrease and magnitudes increase. These results suggest that ice-impact features are more likely to be formed in the downstream sections, due to ice-jam higher level of magnitude.