The North Branch Potomac River flows from Fairfax Stone in West Virginia to its confluence with the South Branch Potomac River near Green Spring, West Virginia, where it turns into the Potomac River proper.

Fairfax Stone Historical Monument State Park is a West Virginia state park commemorating the Fairfax Stone, a surveyor's marker and boundary stone at the source of the North Branch of the Potomac River. The original stone was placed on October 23, 1746 to settle a boundary dispute between Thomas Fairfax, 6th Lord Fairfax of Cameron and the English Privy Council concerning the Northern Neck of Virginia. It determined the proprietorship and boundaries of a large tract of mostly unsurveyed land in the English colonies of Maryland and Virginia.