Ice jams: What are they and what causes them?

Ice jams have been causing concern across New England this winter.

Boston 25 News has been keeping an eye on the one along the Piscataquog River in New Hampshire.

But what exactly is an ice jam and what causes it?

Ice jams occur when the region experiences frigid temperatures and heavy snow, followed by a rapid thaw and heavy rain.

They typically form during the early spring months, but with the right set up, an ice jam can happen at any point during the winter. And the weather this month is to blame for the recent jams. We had a stretch of 13 days where temperatures were below freezing from late December through early January; seven of those days below 20 degrees.

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Ice had formed on many rivers by then and during that cold stretch, a winter storm dumped over a foot of snow. That was followed a week later by temperatures into the 60s, leading to a lot of snow melt. The area also picked up one to three inches of rain during that warmup.

The combination of snow melt with heavy rain causes frozen rivers to swell, which breaks the ice layer on top of the river. The ice layer then breaks into large chunks, which float downstream, but may get stuck in narrow passages, bends in the river or obstructions, like bridges and dams.

The water held back can create flooding upstream, but if the jam suddenly breaks, flash flooding can occur downstream.

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Historically, some of the worst winter floods in New England have been linked to ice jams, and we still have weeks to go before early spring, when we are most at risk.

Cox Media Group