The recent cold weather has had some very interesting side effects. A listener sent a picture of ice circles that had been seen on the Tuttle Creek Reservoir. Question was “How are this formed?” There are two possibilities: ice circles, also known as ice pancakes, which are thin and circular slabs of ice that rotate slowly in the water. It is believed that they form in eddy currents. These circles could form near the intake portion of the dam outlet. Wind can move the circles from where they formed to clusters near the lake shore. By that point, the circles are usually no longer spinning. The other option is ice pans. An expert in river ice from North Dakota, Alan Schlag, states ice pans are more often located in clusters and are formed through solidification of frazil ice onto the outer edge of an already started sheet of ice. The raised edge of the circles is a giveaway that we are actually seeing ice pans.
Mary Knapp, Weather Data Library