The South Branch begins near
Mount Watatic at the outlet of Stodge Meadow Pond in the town of
Ashburnham, Massachusetts and flows through a chain of small lakes (Marble Pond, Ward Pond, and Watatic Pond) before flowing northeast into the town of
Ashby, Massachusetts. North of Ashby, the river enters
New Ipswich, New Hampshire, passes through a small flood-control reservoir, and joins the
West Branch to form the Souhegan River, just north of the intersection known as "Gibson Four Corners".