Enjoy this guest post about an unusual ice phenomenon from naturalist Larry Youmans, who lives in the Flathead Valley:

I had the camera trained on a perfect 40-foot circle of ice.

ice disc 3

This was my second disc sighting, same place, and same disc size. I looked down on the river at the snow-shrouded Glacier Rim Boat Landing, a public access boat ramp. If you stand on the ramp, facing the river, you’ll notice the stream crooks and drops a bit. When the conditions are right, ice discs can form at this spot.

The North Fork of the Flathead River system is a young tributary, a young river with history. One event in its history is the ice disc.

Born about 15,000 years ago during those turbulent times of glaciation, the North Fork has mysteries in its past. Not a long river, it flows south and slightly west along the border of Glacier National Park. As you float this stream there seems to be a series of long pools that slough off to the right at the downstream edge.

Over time, receding glaciers gradually made the North Fork longer and longer. At the boat access, the river flows over bedrock, making this a very old stretch. This stable portion of flow, over time, has probably produced many discs.

River ice circles are an uncommon phenomenon, with sightings ranging globally in colder climates. The few photos of ice discs on other rivers indicate a consistency in stream shape.

ice disc 3

Theories on their formation, and there are many, range from chunks of ice breaking off and grinding in one spot, to alien intervention. It would seem this is a North Fork secret.

Here is one theory: Precipitation changes flow all the time. The build-up of winter snow on the banks reduces the river’s flow. At the boat landing, where the river drops, the right flow can produce a subtle flat swirl or eddy.

As the temperature decreases, it is possible for ice to form in the center of that quiet patch of water. Ice also forms inward from the riverbanks, which causes upstream flow to slow even more. As ice at the center of the swirl expands toward the current, it might start to rotate, forming a perfect circle where it meets the ice growing from the river’s edge.

The reality is there’s no clear understanding of how these circles form. However, I took these photos on Jan. 21, 2014. Historical data indicates Kalispell, Montana, temperatures on Jan. 20 through Jan. 22 ranged from a high of 33 degrees to a low of 12 degrees. The river discharged an estimated 520 cubic feet per second at Glacier Rim on Jan. 21, a common flow rate for the early months of the year.

ice disc

This portion of the North Fork River may have produced many ice circles over the years. I’ve seen two. If I’m lucky enough to see a third, I will spend more time appreciating this rare phenomenon.

~Larry Youmans