The Souhegan River is a tributary of the Merrimack River in the northeastern United States. It is 33.8 miles (54.4 km) long, with a drainage area of 171 sq mi (440 km2), and flows north and east through southern New Hampshire to the Merrimack River.The river begins in New Ipswich, New Hampshire, at the junction of the river's South Branch and West Branch. Flowing northeast, it passes through the center of Greenville and descends through a narrow valley to Wilton, where it turns east. The river valley broadens below Wilton, and the river passes through the center of Milford, crosses the southern portion of Amherst, and enters the town of Merrimack. Shortly upstream of the town center, the river passes over Wildcat Falls, then crosses under the Everett Turnpike and U.S. Route 3 to reach the