It is 33.8 miles (54.4 km) long,[1] with a drainage area of 171 sq mi (440 km2), and flows north and east through southern New Hampshire to the Merrimack River.

Flowing northeast, it passes through the center of Greenville and descends through a narrow valley to Wilton, where it turns east.

It was one of two rivers studied by the New Hampshire Department of Environmental Services to reach data and recommendations about allowable "instream flow".

Tribal fishermen set weirs across the rapids, and then waited and watched for fish, including salmon, alewives, sturgeon, and eels.

Souhegan High School, which serves the towns of Amherst and Mont Vernon, is named after the river.