The South Thompson River in Kamloops B.C. has been attracting attention this week from scientists and curious passersby alike thanks to the appearance of a strange new feature: a large, slowly spinning disc of ice. 

Estimated to be about 40 metres in diameter, the ice disc, which was first spotted earlier this week, is thought to have been caused by a fateful collision of factors, including temperature and the fluid dynamics of the river itself. 

Luke Copland, a professor and holder of the University Research Chair in Glaciology at the University of Ottawa, says large ice discs are formed by a process similar to ice pans (also known as pancake ice), which are a smaller, more common version of the phenomenon found in oceans.

“When water starts freezing, we get frazil ice — fine ice crystals in the water,” he explains. “When the water is turbulent, with currents and waves, that frazil ice starts to form together in clumps, which then collide with each other and start forming these round circular pans.”

In rivers, ice discs tend to form around eddies, where water is flowing upstream against the current due to an obstacle. 

But what sets it spinning? According to one 2016 study, ice discs may begin to spin when warm water sinks below the ice, creating a vortex.

However, Copland says there’s likely a much simpler explanation: the same eddy that created the pan also set it in motion. 

“I expect that the river currents are much more important for this kind of physics effect,” he says.

Watch: Ice disc spins in the South Thompson River