Effects of climate on mid-winter ice jams

The breakup of ice in Canadian rivers and the ensuing ice jams have a multitude of socio-economic impacts. Equally important, but not as well understood, is the strong relationship between the breakup event and the aquatic ecosystem in terms of both habitat and life cycle. Because breakup processes are highly sensitive to weather conditions, there is concern over the potential effects of changing climatic patterns on the ice-jam regime and thus on the stream ecology and local economy. Though breakup commonly occurs in the: spring, it is occasionally triggered by mid-winter 'thaws', which are typical of the more temperate regions of Canada. Mid-winter jams can be more destructive than spring ones and may also have repercussions on the spring event. Current knowledge suggests that small perturbations in winter temperature can produce major changes in the incidence of breakup and ice jams, by altering snowstorms into rainfall events. This expectation is confirmed by a hydroclimatic analysis of field observations and historical data on the upper reach of the Saint John River, which forms the boundary between New Brunswick, Canada and Maine, USA. A slight warming in the past 80 years has been accompanied by a considerable increase in the occurrence of mild winter days, thus contributing to increasing rainfall amounts. This results in augmented flows during the winter, which are lately becoming capable of effecting breakup of the river-ice cover. Implications for future trends in the ice regime of the Saint John River and of other Canadian rivers are discussed. Copyright (C) 2002 John Wiley Sons, Ltd.