Where did that come from?

Q: I recently noticed open water on my normally ice-covered lake. What is the cause?

A: My recollection is that there are periodically similar reports, lakes where areas of open water show up where they haven’t occurred in previous years. I think the most logical explanation is a large influx of groundwater, probably associated with periods of above normal precipitation. Ground water is warm enough (about 50 degrees F) and if the inflow is sufficiently large, the "warm" groundwater plume will rise all the way to the lake’s surface before it cools to 40 degrees F (as long as the water is above about 40 degrees F, it is the least dense layer in the lake and will tend to rise).  If the "warm" ground water plume reaches the surface, the plume either erodes the ice or keeps it from forming. 
— Dave Wright, aquatic ecologist, DNR Ecological Resources