The Back River is a 6.5 miles (10.5 km) river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 located in New Hampshire

New Hampshire

New Hampshire is a state in the New England region of the northeastern United States of America. The state was named after the southern English county of Hampshire. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Atlantic Ocean to the east, and the Canadian...

 and Massachusetts

Massachusetts

The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

 in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. It is a tributary

Tributary

A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...

 of the Powwow River

Powwow River

The Powwow River is a river located in New Hampshire and Massachusetts in the United States. It is a tributary of the Merrimack River, part of the Gulf of Maine watershed.-River course:...

, part of the Merrimack River

Merrimack River

The Merrimack River is a 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...

 watershed

Drainage basin

A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...

. Approximately 3 miles (4.8 km) of the river are in New Hampshire, with the remaining 3.5 miles (5.6 km) in Massachusetts.

The Back River rises in Kensington, New Hampshire

Kensington, New Hampshire

Kensington is a town in Rockingham County, New Hampshire, United States. The population was 2,124 at the 2010 census.-History:Once a parish of Hampton, Kensington was incorporated in 1737 by Massachusetts Governor Jonathan Belcher, when New Hampshire was still part of that colonial province...

 and flows southeast, almost immediately entering the town of South Hampton

South Hampton, New Hampshire

South Hampton is a town in Rockingham County, New Hampshire, United States. The population was 814 at the 2010 census. South Hampton is home to Cowden State Forest and Powwow River State Forest.- History :...

. The river turns south and enters Amesbury, Massachusetts

Amesbury, Massachusetts

Amesbury is a city in Essex County, Massachusetts, United States. Though it officially became a city in 1996, its formal name remains "The Town of Amesbury." In 1890, 9798 people lived in Amesbury; in 1900, 9473; in 1910, 9894; in 1920, 10,036; and in 1940, 10,862. The population was 16,283 at...

, joining the Powwow River at tidewater just downstream from the city's center at the falls of the Powwow.