Patapsco (pətăpˈskō) [key], river, c.65 mi (100 km) long, formed in central Md. by the confluence of the North Branch (c.45 mi/70 km long) and the South Branch and flowing SE into Chesapeake Bay at Baltimore. The lower river is tidal and forms a wide estuary on which Baltimore's harbor installations and industrial suburbs (including Sparrows Point) are located. The Baltimore Harbor Tunnel (1.4 mi/2.3 km long; opened 1957) carries four lanes of vehicular traffic under the estuary; another tunnel, the eight-lane Fort McHenry, opened in 1985. Spanning the waterway is the Francis Scott Key Bridge, built in the 1970s.