Ice circle on American river

A circular ice floe on the US Presumpscot river was formed by a rotating eddy of melted water.

The huge ice floe forms when small ice floes merge, with the circular shape due to rotation which continuously grinds the edges of the floe as it touches the river bed.

Rotating ice circles are rare, and the Presumpscot ice circle was one of the biggest on record, with a diameter of 90 metres, The circles are also known from Piteälven, Sweden.

Lab experiments have shown that small ice circles start to rotate by themselves due to an eddy of meltwater produced under the ice, sinking towards the bottom.