This photo, taken in January of 2017, shows ice discs on the Gananoque River in Ontario, Canada. It was a frigid winter day, and these ice discs had formed in the lee of the current immediately downstream of a dam and waterfall in the river. The ice discs swirled in the current, bumping against each other as new discs formed, likely calving off the patchy ice near the shore. It was a remarkable thing to see on a very chilly walk overtown from my mother’s house.
Ice discs are reportedly an uncommon phenomenon and are the result of rotational shear influencing the formation and shape of ice in cold climate rivers. Some ice discs grow very large, measuring tens of metres in diameter. The discs I photographed in the Gananoque River on that frigid January day were much smaller, averaging about 10 inches (25 cm) across.
It turns out I am not the only person who has noticed ice discs forming on the Gananoque River: these wonderful images of ice disks were taken by photographer Deb Keogh in January of 2025, also on ‘the Crick’ below the dam. Conditions on the Crick that likely lend themselves to the formation of ice disks include the turbidity of water flowing over the dam; the lee in the current immediately downstream; and the shallow basin of this stretch of the Crick—all intersecting with a cold snap. It also seems possible that clear conditions—such as those producing a differential between the temperature of the air and water—may also contribute to the formation of ice discs
Fun fact: A parallel phenomenon is possible, although exceedingly uncommon, in warm climates. El Ojo, or ‘The Eye’ is a floating island composed of organic matter that swirls on its axis in an Argentinean lake. Its slow rotation and shape are reportedly caused by the same rotational shearing effect as is seen with ice disks.
[This oldish photo popped up on Sunday while I was downloading old photos and posts from my Facebork account, which I’ve been dismantling the lazy way, using the helpful ‘Memories’ timeline tool to delete (almost) everything posted on a given date. I’ll likely share a few more of those images here.]