The transition from winter to spring brings warmer temperatures and melting snow.

It’s also ice jam season.

Ice on the rivers doesn’t just disappear. As temperatures warm or river levels come up a bit from snowmelt or rain, the ice breaks up. Most of the time, that ice freely flows downstream. Sometimes, though, the ice gets stuck and jams up the river. You can watch what that looks like in the video player above.

Ice jams tend to happen where the flow slows down, such as a riverbend or narrow spot. Manmade structures such as bridges can also cause jams. North-flowing rivers tend to jam, too, since broken ice is more likely to run into ice held firm by still-cold temperatures.

An ice jam forms on a riverbend. (Wyoming Department of Homeland Security)

When the ice gets stuck, it piles up and essentially creates a dam. The water backs up behind the ice jam and rises. If the ice jam lasts long enough, the river will begin flooding.

Ice jam flooding is almost impossible to predict with much precision. While the odds of ice jams increase with certain conditions and locations, forecasters can’t tell people to expect an ice jam in a certain spot at a certain time.

They can develop suddenly, causing a quick spike in river level. And just as suddenly, an ice jam can break up. Big chunks of ice may become stranded on the ground after being left there by floodwater.

Ice on the banks on the Vermillion River in Ohio. (National Weather Service Cleveland)

Sometimes crews can break up ice themselves to try to stop ice jams before they happen. Check out this video from West Seneca, New York, where they did just that in 2019.

People who live near rivers that ice over in the winter should stay alert as the weather warms. The ice floes drifting downriver are fascinating to watch, but ice jams can crop up with little notice.