in the eastern United States
, starting in the mountains of northern West Virginia
, and running through the Blue Ridge
and past the Shenandoah Valley
(the Shenandoah River
is a major tributary
), before reaching the fall line at Great Falls
and becoming an estuary
of the Chesapeake Bay
. It forms most of the border between Maryland
& West Virginia
The river's name is an Algonquin word, meaning "a trading place"; it was indeed once a major trade route. Before the advent of the railroads, boats were hauling goods up and down the falls (via the C&O Canal) to places like Point of Rocks, MD and Sheperdstown, WV, eventually onward to Cumberland, MD. The remains of the canal still exist as a historical park and hiking trail, though they were heavily damaged in floods in 1996.
The river starts in the West Virginia panhandle, at the junction of the North Branch and the South Branch near Green Spring. The North Branch is the larger of the two, running further west into the West Virginia hills. The Jennings Randolph Lake, an important emergency water source for the Washington area, is on the North Branch. The South Branch follows the Allegheny south into West Virginia, winding through a sometimes very steep valley until it reaches its source in the Monongahela National Forest, near Spruce Knob.
From Green Spring, the Potomac follows a curvy path through the foothills of the Allegheny and the Blue Ridge. At Harper's Ferry, it meets the Shenandoah River; as it approaches Washington, it goes over the sharp rapids of the Great Falls. Fairfax County, Virginia and Washington both pump their drinking water supplies out of the Potomac near the falls. At Washington, the Potomac meets Rock Creek and the Anacostia River; at this point, it becomes a tidal estuary of the Chesapeake Bay. Its mouth on the bay is just off Point Lookout, MD, downstream from St. Mary's City, MD.
There's several crossings over the Potomac, especially as you approach Washington. The Woodrow Wilson Bridge, which is in poor condition and in severe need of its coming replacement, is probably the best known to travellers (since Interstate 95 is routed over it), but other crossings include the Arlington Memorial Bridge, the 14th Street Bridge, and the American Legion Bridge, all of which connect Virginia to Maryland or D.C. The last bridge over the Potomac before its mouth is the Governor Nice Bridge, which carries US Highway 301 between Dahlgren, VA and Charles County, Maryland.