I’ve never seen anything like this! An ice disk is a rare ice formation consisting of a thin, circular sheet of ice. With a bit of water pressure or wind, it moves. In the above video, you can watch one that recently appeared in the Sheyenne River in North Dakota. It’s about 55 feet across and slowly rotates. A hydrologist described the phenomenon:
Allen Schlag, a National Weather Service hydrologist in Bismarck, and Greg Gust, a weather service meteorologist in Grand Forks, said a combination of cold, dense air last weekend and an eddy in the river likely caused the disk.
The cold, dense air — the air pressure Saturday in nearby Fargo was a record high for the city for the month of November, according to Gust — turned the river water into ice, but since the water was relatively warm it didn't happen all at once. Floating bits of ice got caught in the eddy and started to spin in a circle.
You can see photos of more ice disks at Colossal.