To be considered frozen over, lakes Wingra and Monona must have at least half ice cover. Mendota, the largest of Madison’s lakes, is subject to another rule developed by limnologists Edward Birge and Chancey Juday in the early 1900s because of its odd shape: The lake is considered frozen when you can’t row a boat between Picnic Point on UW-Madison’s northeast side to Maple Bluff.