The Arctic air behind a cold front is fresh in both temperature and quality - unlike the air behind a summer cold front. The ice formed from shore to shore to shore overnight behind one such cold front. The sheet of "glass" required that aqua animals needed to break breathing holes to get around and stay alive. One such hole is in the front left of this painting. Air bubbles trapped under the ice must be rich in carbon dioxide from the exhaled breath of the rodents. I suspect this hole was done by a beaver. The family of otters would have punched a riot of holes while the beavers are more solitary swimmers. I am not sure a muskrat would have been big and strong enough to punch such a hole through the ice. There is always something to paint at Jim Day Rapids.