Patapsco River - Geography

Geography

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. The North Branch flows 20.9 miles (33.6 km) southward from its origins in Carroll County. Through most of its length, the Patapsco is a minor river, flowing for the most part through a narrow valley. The last 10 miles (16 km), however, form a large tidal estuary inlet of Chesapeake Bay. The inner part of this estuary provides the harbor of Baltimore, composed of the Northwest Harbor and the Middle Branch. (See Baltimore Inner Harbor.) The Patapsco estuary is south of the Back River and north of the Magothy River.

The Patapsco has a watershed area (including the water surface) of 680 square miles (1,760 km2), or 632 square miles (1,637 km2) of land. Thus, its total watershed area is 7% water.

Patapsco Valley State Park is adjacent to 32 miles (51 km) of the Patapsco and its branches, encompassing a total of 14,000 acres (5,700 ha) in five different areas. The river cuts a gorge 100-200 feet (35-70 m) deep within the park, which features rocky cliffs and tributary waterfalls.

Liberty Dam and its reservoir, located on the North Branch, is a major component of the Baltimore city water system. Besides Baltimore, the river also flows through Ellicott City (the county seat of Howard County) and Elkridge.