The Anacostia watershed is an ecologically and physically diverse system, extending into two physiographic provinces and three political jurisdictions, and containing free-flowing and freshwater tidal segments. The Piedmont province is characterized by relatively narrow and steep-sloped valleys of moderately thin soils, as compared to the undulating Coastal Plain which contains deeper sedimentary soil complexes and supports broader meandering streams. The Fall Line, roughly mirroring the Montgomery County/Prince George's County boundary, delineates the transitional zone between the Piedmont and Coastal Plain provinces. There are three major drainage areas comprising the Anacostia watershed: the Northwest Branch, the Northeast Branch, and the tidal drainage.
The Northwest and Northeast branches are, with the exception of their lowermost reaches, free-flowing (nontidal) streams. The confluence of these two major streams forms the tidal Anacostia River in the vicinity of Bladensburg, Maryland. The tidal drainage area consists of the tidal river and its floodplain, as well as small Coastal Plain streams that flow directly to the tidal river; most of these streams are enclosed in storm sewer systems. The tidal reach of the Anacostia River is 8.4 miles (13.5 kilometers) in length from the confluence of the Northwest and Northeast Branches downstream to the Potomac River. The river joins the Potomac approximately 108 miles (174 kilometers) upstream of the Chesapeake Bay.