This natural phenomenon is known as an ice disc or ice circle, and it’s found in slow moving water in cold environments.
“Ice circles are thin and circular slabs of ice that rotate slowly in the water,” Wikipedia says. “It is believed that they form in eddy currents. It has been shown that existing ice discs can maintain their rotation due to melting.
“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.”