What's that sound near the river?

Recently, while visiting one of FMR's restoration sites — River Oaks Park in Cottage Grove — I noticed that the Mississippi River was completely covered with ice. It was a beautiful yet stark scene: an icy, windswept channel viewed from a bedrock bluff prairie remnant. I happened to be there during the warm-up period following a cold spell brought on by the polar vortex.

As I looked over the river on that sunny afternoon, I heard some strange popping sounds. At first, they were quite faint, but as I listened more closely, they became easily discernible. It sounded like low rumblings mixed with higher-pitched popping and gurgling sounds—an eerie yet compelling auditory experience. Intrigued, I hiked down the bluff to the riverbank and recorded a video capturing some of those sounds.

This phenomenon is sometimes referred to as "ice yowling," a term described by Henry David Thoreau at Flint's Pond in 1850:

the pond began to boom about an hour after sunrise, when it felt the influence of the sun's rays upon it from over the hills; it stretched and yawned like a waking man with a gradually increasing tumult, which was kept up for three or four hours.

The strange sounds from the river ice occur as the ice moves due to expansion and contraction. Additionally, trapped air pockets can slosh around, contributing to the gurgling sounds. The large sheet of ice that caps the river amplifies these sounds. Occasionally, it produces a distant howling or scraping sound — so loud that it has even woken people from their sleep! (Listen to this river ice howl in this clip from MPR.) 

If you're curious, head down to the river and check it out! You might hear the river yowl, or at least gurgle.