The Great Lakes do occasionally freeze over significantly, though never completely. 2013-2014 was one such winter, when ice coverage reached 92% across all five

The Great Lakes do occasionally freeze over significantly, though never completely. 2013-2014 was one such winter, when ice coverage reached 92% across all five lakes. That's quite unusual though. And the city of Toronto uses lake water for cooling. Heat from a downtown cooling loop is dumped into the city's drinking water supply, bringing the temperature up to around 13° from 6° (if I remember correctly).