Most describe "ice-out" to mean a lake or water body is 90% free of ice.
The process of lakes and rivers going through a winter "freeze" and thawing out is very healthy. We call it spring turnover. As water cools in the winter it becomes heavier, dropping to the bottom of the body of water. When it reaches 39 degrees Fahrenheit or 4 degrees Celcius, it begans to turn to ice which is less dense, forcing it back to the top where it freezes. This entire process mixes the water in the lake as well as oxygen and other nutrients to provide a healthy lake.
To look at historical records of Minnesota lake ice out information, check out this site:
Climatology Working Group
The MN DNR also has an ice out page which gives the ice out dates they have received on some of Minnesota's larger lakes.
DNR Ice Out
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