The Merrimack River is a 110-mile (177 km)-long river in the northeastern United States. It rises at the confluence of the Pemigewasset and Winnipesaukee rivers in Franklin, New Hampshire, flows southward into Massachusetts, and then flows northeast until it empties into the Atlantic Ocean at Newburyport. From the point where the Merrimack turns northeast in Lowell, Massachusetts onward, the MassachusettsNew Hampshire border is roughly calculated as the line three miles north of the river.
The Merrimack is an important regional focus in both New Hampshire and Massachusetts. In New Hampshire, the central-southern part of the state is known as the Merrimack Valley Region, and in Massachusetts, the "Merrimack Valley" refers to a cluster of towns and small cities in the northeastern part of the state.
Among its tributaries are the Souhegan River, which extends west from the town of Merrimack, New Hampshire; the Nashua River, which flows north into the city of Nashua; the Concord River, which flows north from Concord, Massachusetts to Lowell; and the Shawsheen River, which after also flowing north, joins the Merrimack at Lawrence.