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New Hampshire Department of Environmental Services

Media Center

DATE: Deember 20, 2018
Shane Csiki (NHDES), (603) 271-2876
Michael Todd (NH HSEM), (603) 223-3641

NHDES and NH HSEM Reminds Granite Staters About Ice Jams

Concord, NH – With the rain expected on Friday, and ice moving on rivers leading to the possibility of ice jams, the New Hampshire Department of Environmental Services (NHDES) and New Hampshire Homeland Security and Emergency Management (NH HSEM) would like to remind Granite Staters regarding ice jams during the winter season. In recent years, New Hampshire has had extended episodes of subfreezing weather, which have then been followed by large, rapid warm ups, often combined with heavy rain storms. These events led to breakup of the ice cover into large chunks that flowed downstream, and then just as quickly froze in place when temperatures tumbled, creating ice jams. Jams have led to flooding behind them, such as in Plymouth in 2017. As water backs up behind a jam, flooding can occur rapidly, damaging property, inundating roads and leaving people cut off.

While ice jams are of greater concern in February and March, they can occur in late December or January under the right conditions. Although weather plays a key role in ice jam formation, bends, narrows or shallow, flat portions of rivers or barriers such as bridge piers, can enhance the potential for ice jams to develop. Jams can break rapidly, and can lead to downstream flash flooding, as the stored water behind the jam flows quickly downstream.

Every ice jam is unique. Where an ice jam will form, how long it will last, and if and how much flooding behind it will occur cannot be predicted, as observed in Franconia in 2016, which caused flooding of a road, leaving home access cut off, with inundation of one home.

If you see an ice jam forming, you should notify your town or city emergency management director. Though many ice jams throughout New Hampshire form and break without causing a problem, local emergency personnel should know about them.

The Cold Regions Research and Engineering Laboratory partnered with the state and National Weather Service, Gray, ME last year to deliver information regarding ice jam formation and techniques to address them. A copy of that training, where you can find more information, can be found here: Adobe Acrobat Reader Symbol.



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NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
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