Geography of the River

Around 10 to 20 million years ago the Atlantic Ocean withdrew from the Atlantic Coastal Plain leaving behind small eastward moving streams . Uplift of the Piedmont Plateau and the Appalachian Highlands increased the slope of the land causing streams to deepen their valleys and coalesce into a river, creating the Potomac. 2 million years ago the Potomac succeeded in carving out the broad valley that it currently flows through. ¹

The Potomac River now begins with the convergence of two branches, North and South, just east of Greenspring, Hampshire County, West Virginia. in the Appalachian Mountains. The North Branch of the Potomac begins in Fairfax Stone, West Virginia and runs northeast for 95 miles forming the boundary between Maryland and West Virginia. The South Branch of the Potomac starts in northern Highland County, West Virginia and stretches for 130 miles. ²

Continuing from the convergence of the North and South branches, the Potomac River meanders for 383 miles before reaching the Chesapeake Bay. ² It has a drainage area of 14,670 sq mi over 4 states including, West Virginia, Maryland, Pennsylvania, and Virginia, and the District of Columbia. ³

The largest flow ever recorded on the Potomac River was in the Washington D.C. area in March of 1936,  reaching 275 billion gallons of water per day. In the same region, the lowest flow ever recorded was 388 million gallons of water per day in September 1966. In the district the average flow is 7 billion gallons of water per day, with 486 million gallons of water withdrawn every day for the local water supply. ³



1. U.S. Geological Survey. "USGS: The River and The Rocks (The Origin of the Potomac River Valley)." The River and The Rocks (The Origin of the Potomac River Valley). N.p., 1 Mar. 2005. Web. 10 Dec. 2014. <http://cr.nps.gov/history/online_books/grfa/sec4.htm>.
2. "Potomac River (river, United States)." Encyclopedia Britannica Online. Encyclopedia Britannica, 1 July 2013. Web. 9 Dec. 2014. <http://www.britannica.com/EBchecked/topic/472764/Potomac-River>.
3. "General Facts & FAQs." Interstate Commission on the Potomac River Basin, n.d. Web. 8 Dec. 2014. <http://www.potomacriver.org/facts-a-faqs/faqs>.