Ice Circle spinning on Middle Fork Snoqualmie River from Kaylyn Messer on Vimeo.
Meteorologist Morgan Palmer explained that ice circles are pretty simple, actually.
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"In a river or stream, there are currents and eddies. In an eddy, water flows in a circular motion because of obstructions, whether they be boulders or just the shape of the stream itself," Palmer said.
If the eddy is persistent, the water in the center of the eddy is more or less “trapped” and is susceptible to freezing.
The freezing will start at the center of the eddy and work out to the edges where, eventually, the water is flowing too fast to freeze. Or, in other cases, it will happen when a sheet of ice breaks off.
This creates a disc of ice that can spin due to the motion of the surrounding rushing water!
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