The Souhegan River is about 35 miles long. It flows north from Massachusetts into New Hampshire and then east to a confluence with the Merrimack River, passing through a half-dozen towns, including the center of Milford, New Hampshire

The river is used for water supplies, hydropower, and recreation. There are 28 threatened or endangered species sharing the watershed with 35,000 people. It is part of the New Hampshire Rivers Management Protection Program.