• The Potomac River's watershed area includes parts of Virginia, West Virginia, Maryland, and Pennsylvania.
• Flows from the Valley and Ridge to the Blue Ridge, then Piedmont, and the Coastal Plain where it reaches the Chesapeake Bay.
• Over much of its course, it forms the southwestern state boundary of Maryland where it borders West Virginia and Virginia.
• In Fairfax County, it forms spectacular waterfalls at Great Falls. Here, the river is eroding resistant metamorphic rocks of the Piedmont at the Fall Zone.
Great Falls Park, Fairfax County. (Photograph by Robert Whisonant)
• The Potomac's major Virginia tributary is the Shenandoah River. The confluence of the two rivers is at Harpers Ferry, where the Potomac flows through a water gap through the Blue Ridge. The Shenandoah joins the Potomac just west (upstream) from the water gap.
• In Virginia, the Shenandoah River flows along the "Great Valley". At Front Royal, two main forks of the river merge. The North Fork flows on the northwest side of Massanutten Mountain where it parallels I-81. The South Fork flows on the southeast side of Massanutten Mountain.