Hydro-climatic analysis of mechanical breakups reconstructed from tree-rings, Necopastic watershed, northern Québec, Canada

We used a 54-year (1950–2003) ice scar chronology constructed from damaged trees to investigate relationships between large scale hydro-climatic conditions and mechanical breakup magnitude in the Necopastic River watershed (James Bay, northern Québec). Our objectives were: (1) to identify hydrologic and climatic variables that explain variations in mechanical breakup magnitude at the watershed scale, (2) to organize these variables in terms of importance and (3) to construct a predictive model for ice-floods in the Necopastic watershed. We used parametric correlation analysis to measure the degree of linear association between variables and classification trees (CT) coupled with a cross-validation approach to construct the predictive model and to organize variables according to their importance. In the Necopastic watershed, the type and magnitude of breakups are determined by an interaction between forces that maintain the ice cover in place and forces that tend to dislodge it. In contrast with lakes, early and rapidly rising floods, rather than abnormally high flood discharge, create conditions that favour intense mechanical breakups. Moreover, cold and snowy spring conditions during high positive Arctic Oscillation (AO) indices delay the thermal degradation of the ice cover and preserve its mechanical properties over longer time periods.