Large spinning ice circle in North Dakota

[Alps]

It's just really awesome.

http://news.yahoo.com/unusual-ice-circle-forms-north-dakota-river-170950600.html

[Dr Frankenstein]

I remember watching a short documentary about this phenomenon. They're still researching it.

[mgk920]

I recall seeing a spinning ice disk on the Wolf River in northern Wisconsin (just upstream from the Menominee Indian Reservation) while out winter camping with the Boy Scout troop back when I was in high school.  Unfortunately, nobody brought a movie camera along for that weekend.

Mike

[Alps]

I remember watching a short documentary about this phenomenon. They're still researching it.

Seems fairly straightforward - it's a natural eddy in the river, ice gathers on the surface due to cold, dense air freezing the water, and the ice gets sucked into the eddy. When the air gets colder, the "ice cubes" (per the article) freeze together into a disk, but the underlying eddy keeps moving, so it never freezes to the ice around it. (Or it does, but then the warmer air the next day melts the fringe and it breaks away to float freely again.)