Ice jams occur every winter in New Hampshire, creating potential flooding hazards.
Winter flood hazards on New Hampshire’s rivers and streams involve ice jams. Ice forms on cooling rivers in early winter. When the ice breaks up and starts to flow with warmer weather, often with heavy rain, jams can develop. When water backs up behind jams, sometimes quickly, flooding can occur upstream and downstream of the jam. While ice jams can form almost anywhere along a channel, making them difficult to predict, past jam locations are places to watch for signs of future jam development. Certain river features, such as sharp bends or a narrowing of the stream channel, can also increase the potential for formation. Recent ice jams that have caused flooding issues in New Hampshire include the Gale River at Franconia (2016) and the South Branch Piscataquog River at New Boston (2017 and 2018).
If a community experiences an ice jam, staff within NHDES can apply expert knowledge of river processes to help diagnose the site-specific causes and evaluate risks to adjacent land and infrastructure, as needed. Technical expertise can also be provided to discuss monitoring options during an ice jam event. Historical flood information is available via a statewide Flood Hazards Geodatabase, maintained by the New Hampshire Geological Survey (NHGS). The Dam Bureau is a source of historical ice jam information.
Both the Dam Bureau and NHGS maintain partnerships with federal agencies, through the New Hampshire Silver Jackets, that can provide resources and assistance regarding ice jams. In 2017, the New Hampshire Silver Jackets completed an ice jam outreach project, for which the USACE Cold Regions Research and Engineering Laboratory (CRREL) provided ice jam training sessions statewide. See their entire presentation package, which contains an abundance of information regarding all aspects of ice jam. Further options regarding ice jams can be found in the New Hampshire Flood Hazards Handbook: A Guide for Municipal Officials.