Welcome To The (Ice) Jam: N.H. Officials Say Freeze-Thaw Cycle Could Bring Floods

Dec 21, 2018

Ice jam season is arriving in New Hampshire, and officials are warning residents and town officials to be on the lookout for potential hazards.

An ice jam can form when rain falls on a frozen river or stream amid mild temperatures. The ice can partially melt, the water level rises and the ice breaks into chunks.

Those flow downstream and get caught in narrow river bends or under bridges, forming a blockage that can lead to flooding.

State flood hazards program administrator Shane Csiki says jams may be more severe in January and February, when ice is thicker – or in the Northern and Western parts of the state, where steep terrain creates more constrictions where ice can jam.

But they can occur any time in the freeze-thaw cycle, and he says conditions are perfect right now for ice to break up into jams.

"It's not as thick now as when it would be in January or February,” he says. “But there has been enough of an ice cover forming on rivers."

He says residents should let their local emergency officials know if they see a jam or suspect one is forming.

"And then if something becomes a real problem ... they know who at the state that they can contact,” Csiki says.

This lets the town prepare for possible flooding or, more rarely, take steps to remove the jam. Severe ice jams can cause millions of dollars in damage and even be deadly.

The limited data collected by federal officials says New Hampshire is 13th in the nation for reports of ice jams, with nearly 700 recorded in 85 of the state’s rivers since the 1800s.

The data suggests ice jams are most common in the larger Plains states. But it also suggests reports of ice jams are on the rise in New Hampshire.

Any increase could be tied to conditions that could occur more often with climate change as the region's climate becomes wetter, warmer and more volatile.

Slide the center line of the graphic below to see the change over time in number of reported ice jams in New Hampshire, as recorded by the U.S. Army Corps of Engineers. Darker spots mean more ice jams have reportedly occurred in that location. This data is not necessarily complete. 

Graphic by Rebecca Lavoie, with data from U.S. Army Corps