The Potomac River runs 405 miles (652 km) from Fairfax Stone Historical Monument State Park 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 daily flow during the water years 1931–2018 was 11,498 cubic feet (325.6 m³) /s.
The highest average daily flow ever recorded on the Potomac at Little Falls, Maryland (near Washington, D.C.), was in March 1936 when it reached 426,000 cubic feet (12,100 m³) /s.
The lowest average daily flow ever recorded at the same location was 601.0 cubic feet (17.02 m³) /s in September 1966 The highest crest of the Potomac ever registered at Little Falls was 28.10 ft, on March 19, 1936;
however, the most damaging flood to affect Washington, DC and its metropolitan area was that of October 1942.

The river has two sources.
The source of the North Branch is at the Fairfax Stone located at the junction of Grant, Tucker, and 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.
As it flows from its headwaters down to the Chesapeake Bay, the Potomac traverses five geological provinces: the Appalachian Plateau, the Ridge and Valley, the Blue Ridge, the Piedmont Plateau, and the Atlantic coastal plain.

Once the Potomac drops from the Piedmont to the Coastal Plain at the Atlantic Seaboard fall line at Little Falls, 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.
The Potomac River runs 405 miles (652 km) from Fairfax Stone Historical Monument State Park 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 daily flow during the water years 1931–2018 was 11,498 cubic feet (325.6 m³) /s. The highest average daily flow ever recorded on the Potomac at Little Falls, Maryland (near Washington, D.C.), was in March 1936 when it reached 426,000 cubic feet (12,100 m³) /s. The lowest average daily flow ever recorded at the same location was 601.0 cubic feet (17.02 m³) /s in September 1966 The highest crest of the Potomac ever registered at Little Falls was 28.10 ft, on March 19, 1936;
however, the most damaging flood to affect Washington, DC and its metropolitan area was that of October 1942.
The river has two sources. The source of the North Branch is at the Fairfax Stone located at the junction of Grant, Tucker, and 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. As it flows from its headwaters down to the Chesapeake Bay, the Potomac traverses five geological provinces: the Appalachian Plateau, the Ridge and Valley, the Blue Ridge, the Piedmont Plateau, and the Atlantic coastal plain.
Once the Potomac drops from the Piedmont to the Coastal Plain at the Atlantic Seaboard fall line at Little Falls, 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.
Potomac River