In a study funded by the National Science Foundation (NSF) and published in the September 2007 issue of the journal Limnology & Oceanography, North Temperate Lakes (NTL) LTER and University of Wisconsin at Madison researchers John Magnuson, Olaf Jensen and Barbara Benson reported that winter ice cover over lakes, rivers, and ponds in the region are lasting fewer and fewer days.
The researchers used long-term ecological research records of lake and river ice records spanning 150 years from sources as diverse as newspaper archives, transportation ledgers and religious observances. According to Magnuson, the formation and breakup of ice are important seasonal events in mid- to high-latitude lakes and rivers. Thus, he said, "The timing of these events--ice phenology--is sensitive to the characteristics of individual waterbodies and to broader-scale weather patterns and climate variability."
You can read a National Science Foundation report of the research here