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I've seen ice do a lot of strange things, but I've never seen it do this before: Ice Circle spinning on Middle Fork Snoqualmie River (Vimeo).

ice circle

What causes ice circles to form?

  • See also Earth Science SE, which I believe would be a better fit for this question. – gerrit Jan 10 '17 at 11:26
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Ice Disk

That phenomenon is called an ice disc or ice circle. It occurs in slow moving water in cold climates.

Ice discs form on the outer bends in a river where the accelerating water creates a force called 'rotational shear', which breaks off a chunk of ice and twists it around. As the disc rotates, it grinds against surrounding ice — smoothing into a circle.

From Wikipedia - Ice Circle

Your Ice Circle

This particular ice disk (from your video), was formed because there was a certain climate which is perfect for such phenomenon to build.

"Calm winds this week combined with 6 mornings in a row in the low 20s made for perfect conditions to form the circles," said Q13 Chief Meteorologist Walter Kelley.

From Fox4KC

Why is it so rare?

(...) but I've never seen it do this before(.)

The occurrence of such are absolutely rare since you have to face perfect really specific conditions (water, climate, river etc.).

Ice circles or disc are a relatively uncommon phenomenon. One of the earliest recordings is of a slowly revolving disc spotted on the Mianus River in 1895.

From Fox4KC

An unusual natural phenomenon, ice disks occur in slow moving water in cold climates and can vary in size, with circles more than 15 metres (49 ft) in diameter observed.

From Wikipedia - Ice Circle

Further reading

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