The Susquehanna River forms from two main branches: the North Branch, which rises in Cooperstown, New York, and is regarded by federal mapmakers as the main branch or headwaters,[11] and the West Branch, which rises in western Pennsylvania and joins the main branch near Northumberland in central Pennsylvania.
The river drains 27,500 square miles (71,000 km2), including nearly half of the land area of Pennsylvania. The drainage basin includes portions of the Allegheny Plateau region of the Appalachian Mountains, cutting through a succession of water gaps in a broad zigzag course to flow across the rural heartland of southeastern Pennsylvania and northeastern Maryland in the lateral near-parallel array of mountain ridges. The river empties into the northern end of the Chesapeake Bay at Perryville and Havre de Grace, Maryland, providing half of the Bay's freshwater inflow. The bay lies in the flooded valley, or ria, of the Susquehanna.
^U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National MapArchived March 29, 2012, at the Wayback Machine, accessed August 8, 2011