Potomac River - Geography

Geography

The river forms part of the borders between Washington, D.C., and Virginia on the left descending bank and Maryland and Virginia on the river's right descending bank. The majority of the lower Potomac River is part of the State of Maryland. Exceptions include a small tidal portion within the District of Columbia, and the border with Virginia being delineated from "point to point" (thus various bays and shoreline indentations lie in Virginia). Except for a small portion of its headwaters in West Virginia, the North Branch Potomac River is considered part of Maryland to the low water mark on the opposite bank. The South Branch Potomac River lies completely within the state of West Virginia except for its headwaters, which lie in Virginia.

The Potomac River runs 405 miles (652 km) from the Fairfax Stone in West Virginia on the Allegheny Plateau to Point Lookout, Maryland, and drains 14,679 square miles (38,020 km2). The length of the river from the junction of its North and South Branches to Point Lookout is 302 miles (486 km). The average flow is 10,800 ft³/s (306 m³/s). The largest flow ever recorded on the Potomac at Washington, D.C., was in March 1936 when it reached 425,000 ft³/s (12,000 m³/s). The lowest flow ever recorded at the same location was 600 ft³/s (17 m³/s) in September 1966.

The river has two sources. The source of the North Branch is at the Fairfax Stone located at the junction of Grant, Tucker, Preston counties in West Virginia. The source of the South Branch is located near Hightown in northern Highland County, Virginia. The river's two branches converge just east of Green Spring in Hampshire County, West Virginia, to form the Potomac.

Once the Potomac drops from the Piedmont to the Coastal Plain, tides further influence the river as it passes through Washington, D.C., and beyond. Salinity in the Potomac River Estuary increases thereafter with distance downstream. The estuary also widens, reaching 11 statute miles (17 km) wide at its mouth, between Point Lookout, Maryland, and Smith Point, Virginia, before flowing into the Chesapeake Bay.