||This article needs additional citations for verification. (August 2011)|
Patapsco Valley State Park at Catonsville.
|- left||North Branch|
|- right||South Branch|
|Cities||Baltimore, Elkridge, Ellicott City|
|- elevation||260 ft (79 m)|
|- elevation||0 ft (0 m)|
|Basin||632 sq mi (1,637 km2)|
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 name "Patapsco" is derived from the Algonquian pota-psk-ut, which translates to "backwater" or "tide covered with froth."
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 including Thoms Cove. (See Baltimore Inner Harbor.) The Patapsco estuary is south of the Back River and north of the Magothy River.
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.
On the 1612 John Smith map, it was called the Bolus River. As the river was not navigable beyond Elkridge, it was not a major path of commerce. The Patapsco valley was used as the route of the Baltimore and Ohio Railroad's original main line west constructed from 1829 onwards, and this route remains, though much altered. Many old railroad bridges were constructed in the valley, most notably the Thomas Viaduct and the Patterson Viaduct, of which ruins remain. Flour mills and a hydropower dam were formerly powered by the river.
The valley is prone to flooding, though at long intervals. The most recent severe flood occurred in 1972 as a result of rainfall from the remnants of Hurricane Agnes, in which Ellicott City and the Old Main Line sustained serious damage.
The mouth of the Patapsco River forms Baltimore harbor, the site of the Battle of Baltimore during the War of 1812. This is where Francis Scott Key, while aboard a British ship, wrote "The Star-Spangled Banner," a poem later set to music as the national anthem of the United States. Today, a red, white, and blue buoy marks the spot where the HMS Tonnant was anchored.
Water quality 
The eastern portion of the Patapsco River is in a highly urbanized area and is subject to extensive stormwater runoff and other forms of water pollution. The Maryland Department of the Environment has identified the Lower North Branch as containing high levels of heavy metals (chromium, arsenic, cadmium, copper, mercury, nickel, lead, selenium, and zinc), as well as phosphorus, fecal coliform bacteria, and PCBs.
Clean-up efforts by the residents of surrounding communities have been led by environmental nonprofit organizations, such as The Friends of Patapsco Valley & Heritage Greenway, Inc. (PHG). From 2006 to 2012, PHG volunteers participated in 183 stream clean ups, removing 264 tons of trash from the streams of the Patapsco Valley watershed.
See also 
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- U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map, accessed April 1, 2011
- "Ghosts of industrial heyday still haunt Baltimore’s harbor, creeks". Chesapeake Bay Journal. Retrieved 2012-09-08.
- Baltimore County Department of Environmental Protection and Resource Management. Towson, MD. "Patapsco River Watershed." 2009-07-13.
- Maryland Department of the Environment. Baltimore, MD (2009). "Notice of Intent to Establish Total Maximum Daily Load (TMDL) of Sediment: Patapsco River Lower North Branch Watershed."
- "Protecting the Valley". Friends of Patapsco Valley & Heritage Greenway, Inc. Retrieved May 24, 2012.
- Maryland DNR's Surf Your Watershed: Patapsco/Back River
- Maryland Port Administration
- Blue Water Baltimore, formed in 2010 from a merger of the Gwynns Falls, Jones Falls, Herring Run, and Baltimore Harbor Watershed associations, and the Baltimore Harbor Waterkeeper
- Friends of Patapsco Valley and Heritage Greenway
- Patapsco River Rock Building