What's that sound near the river?

View of ice-covered Mississippi River from River Oaks Park in Cottage Grove, MN

Photo: Joe Walton, FMR

Recently, while out at one of FMR's restoration sites, River Oaks Park in Cottage Grove, I noticed that the Mississippi River was completely covered with ice. What a beautiful yet stark scene that was: an icy and windswept channel as viewed from a bedrock bluff prairie remnant. I happened to be there during the warm up period, which followed the cold spell we had from the polar vortex.

As I was looking over the river that sunny afternoon, I heard some strange popping sounds. At first they seemed rather faint, but then as I listened more closely, they actually were easily discernible. It sounded like low rumblings mixed with sort of higher pitched popping and gurgling; an eerie yet compelling sound. Intrigued, I hiked down the bluff to the river shore and took a video of the river, which captured some of the sounds.

Sometimes known as ice yowling, this phenomenon was 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 river ice are due to the movement of the ice as it expands or contracts. Also, air pockets may get trapped and slosh around, adding to the gurgling sounds. The sounds are amplified and carried by the large sheet of ice that caps the river. Sometimes it can make a distant howling or scraping type sound, which can be so loud that it has actually woken people up from their night's sleep!

If you are curious, get yourself down to the river and check it out! Maybe you will hear the river yowl, or at least gurgle.