The most common natural hazard in New Hampshire is flooding. Every year some part of the state experiences flash flooding of smaller rivers and streams or poorly-drained urban areas, main stem river flooding, coastal flooding or a combination of the three.
The most recent series of floods began in October 2005 with a flood that primarily affected the southwest corner of the state and devastated the town of Alstead. The flood killed seven people. It was followed by floods in May 2006 and April 2007 and a series of floods during the late summer and early fall of 2008.
These floods all had one thing in common - continuous heavy rain caused by two or more weather systems that stalled over the state. Because of the state's rough topography, its many small rivers and streams can quickly overflow their banks during heavy, continuous rain. There is no place for the excess water to go except onto roads and fields and into populated areas.
Historically, the state's two largest floods occurred in 1936 and 1938. The 1936 flood was associated with snow melt and heavy precipitation. The 1938 flooding was caused by the Great New England Hurricane of 1938. Those floods prompted the construction of a series of flood control dams, built in the 1950s and '60s. They continue to be operated by the U.S. Army Corps of Engineers.
The threat of flooding is most common in the spring from a combination of snow melt and rain. But floods may strike the state at any time of the year. Even during a drought! All it takes is a torrential downpour or a sustained period of rain.
Flood water is very dangerous. Its force can undermine road surfaces and sweep buildings away. It is also very likely to be contaiminated with petroleum products, sewage, agricultural chemicals. Never ingest or touch flood water and never swim in it or go boating on it. Floods are dangerous. They are not a recreational opportunity.
The links below provide further information on flood safety and recovery.