Ice circle seen on the Ram River, AB (Canada), circa 1997
Each year, people around the world report seeing these “ice circles” or ice discs during river freeze-up. These typically occur near the outside of river bends. The stream currents have to curve around the river bend and, if there is an ice cover in the bend, the curving flow creates a curved drag force on the underside of the ice. If the ice is really thin and frail, then the ice cover will break up and be washed away with the flow. If the ice cover is thick and strong, then it will withstand this rotational force and nothing will happen. However, sometimes, there is just the right balance between the rotational drag force created by the curving flow and the ice thickness/strength and, in such very special cases, an ice disc forms. In this case, the ice is weak enough to be fractured, but not weak enough to break up completely. If conditions are ‘just right’ then you can get part of the ice rotating, and as it rotates it grinds against the remaining (shorefast) ice – rounding the edges and making the disk almost perfectly round.
Ice discs can also form when frazil pans and rafts get caught in a river eddy. As more and more pans are trapped, they freeze together and eventually form one very large spinning raft. If you would like to see the early stages of this – check out this post.
Ice experts in Canada affectionately refer to these ice discs as “pizza ice”.
(photo by Richard Brown, Pacific Northwest National Laboratory, Ecology Group)