The Patapsco River is a 39-mile-long (63 km) river in central Maryland which flows into Chesapeake Bay. The river's tidal portion forms the harbor for the city of Baltimore. With its South Branch, the Patapsco forms the northern border of Howard County, Maryland.
The Patapsco proper begins at the confluence of the North and South Branches, near Marriottsville, approximately 15 miles (24 km) west of downtown Baltimore. The 19.4-mile-long (31.2 km) South Branch rises further west at Parrs Spring, where Howard County, Carroll, Frederick, and Montgomery counties meet.