Data includes day of freeze-up and thaw dates of seven northern primary lakes (Allequash, Big Muskellunge, Crystal, Sparkling, Trout, unnamed 27-02 [Crystal Bog] and 12-15 [Trout Bog]) as well as historic data from Little Rock Lake. Observations are made approximately every other day during times of freeze and thaw. A lake is considered ice covered when the sampling station (the deepest part of the lake) is ice covered. The lake is considered thawed when it is possible to drive a boat from the boat landing to the sampling station without encountering ice. Sampling Frequency: annually. Number of sites: 8.