the potomac river is a river in the eastern united states. it is 665 kilometers long. it forms part of the borders between maryland, west virginia, washington, d.c., and virginia. the potomac flows to the chesapeake bay. it has two sources. the north branch starts at fairfax stone in west virginia, and the south branch starts in highland county, virginia. the north branch and the south branch meet in hampshire county, west virginia, and make the potomac river.