The West Branch Susquehanna River is one of the two principal branches, along the with North Branch, of the Susquehanna River in the northeastern United States. The North Branch, which rises in upstate New York, is sometimes regarded as an extension of the main branch, with the shorter West Branch being its principal tributary. The West Branch, which is 160 mi (257 km) long, is entirely within the state of Pennsylvania, draining a large mountainous area within the Allegheny Plateau in the western part of the state. Along most of its course it flows in tight meanders along mountain ridges and through water gaps, forming a large zigzag arc through central Pennsylvania around the north end of the Allegheny Ridge. In colonial times the river provided an important water route to the Ohio River valley. In the 18th century, its lower valley became a significant industrial heartland of Pennsylvania.
The eastern side of Williamsport, seen from the southeast on Bald Eagle Mountain. In the foreground is the west branch of the Susquehanna River
. Beyond Williamsport can be seen the higher part of the dissected Allegheny Plateau
The West Branch rises on the west side of the Alleghenies, in northwestern Cambria County, approximately 20 mi (32 km) east of the city of Indiana. It flows generally north past Cherry Tree, into southern Clearfield County. Near Mahaffey it turns generally northeast and flows past Clearfield . In northern Clinton County it turns to the southeast to Lock Haven. At Lock Haven it turns east, flowing past Williamsport, the largest city on the river, then turns south, emerging from a watergap between Mountour Ridge and Shamokin Mountain and joining the North (Main) branch of the Susquehanna from the northwest approximately 2 mi (3 km) north of Sunbury.
In its upper reaches it receives Chest Creek from the south at Mahaffey, Clearfield Creek from the south 2 mi (3 km) east of Clearfield, and Sinnemahoning Creek from the northwest at Keating. In its lower reaches it receives Bald Eage Creek from the southwest at Youngdale, Lycoming Creek from the north and Loyalsock Creek from the northeast at Williamsport, and Muncy Creek from the northeast at Muncy
Up through the early 19th century the river provided the principal canoe route across the Alleghenies connecting the Susquehanna and Ohio valleys, with a portage at Cherry Tree to Blacklick Creek, a tributary of the Conemaugh River. In the late 18th century, Cherry Tree marked the frontier between the Pennsylvania Colony and the Shawnee and Lenape lands to the west as specified by treaty.