Potomac River

River 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 below Washington. It forms most of the border between Maryland and Virginia & 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.

I have seen many maps which delineate the boundary between Maryland and Washington, DC on one side and Virginia and West Virginia on the other as going down the center of the Potomac River.  These maps are incorrect.

In the 1629 Charter of Maryland¹ , English King Charles I granted² Cecilus Calvert, second Lord Baltimore.

"...all that tract of land...unto the true Meridian of the first fountain of the River of Pattowmeck, and from thence trending toward the South unto the farther banke of the fore-said river, and following the West and South side thereof unto a certaine place called Cinquack, situate neere the mouth of the said River, where it falls into the bay of Chesopeack..."

making the south bank of the river the boundary, putting the entire river inside Maryland.  This description was open to some interpretation:

• The "first fountain" of the Potomac had not actually been located at the time.
• The Potomac divides into a "North Branch" and a "South Branch" a little bit below Cumberland.  A considerable amount of territory (4,300 square miles) lay between the two.
• The lower Potomac is actually an estuary, and its water level rises and falls (and its banks consequently shift) with the tide.  Tributary estuaries come in from Virginia and the points where one leaves off and the main river begins are vague at best.

These and other factors caused boundary disputes between Maryland and Virginia (and later West Virginia) which were not resolved until 1908.  Maryland had been founded as a haven for Roman Catholics, and early on, anti-Catholic sentiment guaranteed that Maryland was on the losing end of these disputes.  Most importantly, in 1736 a stone was placed on the North Branch of the Potomac and declared the river's "first fountain".

This boundary's definition has resulted in some odd situations in the 20th Century:

• In the early 20th Century³, WESTVACO diverted the Potomac to the other side of its paper mill, suddenly putting the town of West Piedmont, West Virginia on the north side.  It took a ruling of the Supreme Court to declare that West Piedmont, West Virginia was now Luke, Maryland.
• During the first half of the 20th Century, Maryland was actually a haven for gambling. Slot machines were illegal in Virginia, but enterprising Virginians with waterfront property took advantage of the border's definition by building long piers out into the river and building casinos on the piers.  The authorities could do nothing as the casinos were legally in Maryland.

Maryland and Virginia still dispute use of the lower Potomac (now, mostly for fishing, oystering and crabbing) to this day.

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¹http://www.mdarchives.state.md.us/msa/educ/exhibits/founding/html/charter.html
²granted, Charles didn't own it to begin with, but that never stopped him.
³Luke, Maryland was chartered in 1922; I'm guessing it happened then or the previous year.