POTOMAC RIVER drains the eastern slopes of the central Allegheny Mountains into Chesapeake Bay. Two main streams, the East and West Branches, unite to form the upper Potomac, which is joined by the Shenandoah River at Harpers Ferry. A freshwater river for 287 miles, the Potomac below Washington, D.C., is a tidal estuary 125 miles long and between two and eight miles in width. The region, especially the lower portion, was home to many Native Americans. Spaniards probably reached the Potomac estuary before 1570, and it was mapped by Captain John Smith in 1608 at the time of initial English settlement in Virginia. Agents of George Calvert probably explored the upper Potomac. After the founding of Maryland in 1634, the Potomac was the early passageway for the colony. Owing to Great Falls above Washington, rapids at Harpers Ferry, and Indian problems, the upper Potomac was long unimportant. In the 1720s, Tidewater Virginians, Germans, and Scots-Irish crossed it into the Shenandoah Valley, and about 1740 Thomas Cresap, a militant Marylander, settled at Oldtown above the junction of the South Branch in western Maryland. Slowly, the Potomac Valley became a pathway to the Ohio Valley, used by the Ohio Company of Virginia, by George Washington, and by General Edward Braddock. Later enterprises using the route were the Patowmack Company of 1785, the Cumberland Road of 1807 (later extended westward as the National Road), the Baltimore and Ohio Railroad of 1827, and the Chesapeake and Ohio Canal Company of 1828. The canal competed with roads and railroads until the early 1900s. The Potomac formed the effective eastern border between the Union and the Confederacy and was the scene of many campaigns and crossings including Antietam.
Gutheim, Frederick A. The Potomac. New York: Rinehart, 1949. One of the excellent Rivers of America series.
Potter, Stephen R. Commoners, Tribute, and Chiefs: The Development of Algonquin Culture in the Potomac Valley. Charlottesville: University Press of Virginia, 1993.
Sanderlin, Walter S. The Great National Project: A History of the Chesapeake and Ohio Canal. Baltimore: Johns Hopkins Press, 1946.