According to Wikipedia, the phenomenon of perfect spinning ice circles in winter rivers is rare, which means the two documented on social media in Michigan rivers recently is pretty exceptional.

Ice circles are thin spheres that form in lakes and rivers when a chuck of ice breaks free and is rotated by the water currents and ground into a perfect circle.

Two videos have been taken recently in Michigan rivers that demonstrate that either the phenomenon is not as rare as originally thought, ot that the conditions for it have been damn near perfect recently.

The first video was taken by Karla Dahms last month on the Muskegon River near Leota. The spinning disc here appears quite large and was featured on the AccuWeather video page.

And then this weekend, Jason Robinson of Vestaburg recorded a similar sphere spinning in the Pine River near his home. This post on Facebook prompted a MLive article highlighting the cause of the phenomenon.

According to Wikipedia:

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.[7] As the disc rotates, it grinds against surrounding ice — smoothing into a circle.[2] A relatively uncommon phenomenon, one of the earliest recordings is of a slowly revolving disc spotted on the Mianus River and reported in an 1895 edition of Scientific American.