THE RIVER AND THE ROCKS The Geologic Story of Great Falls and the Potomac River Gorge

The Great Falls of the Potomac River is the most spectacular landscape feature of the Washington metropolitan area, and it has influenced the lives and fortunes of man in the Potomac Valley for centuries. For countless generations of Indians it was a place to gather, to trade, and to fish. For the early settlers it was a barrier to river navigation, an obstacle that canal builders struggled to overcome. For the past 100 years the river above the falls has been the principal source of water for the city of Washington.

In its seaward course, the Potomac River crosses many small rapids and cascades, but these are insignificant in comparison with the foaming fury of Great Falls, where the river drops 40 feet in about 200 yards and is channeled into a narrow rock-walled gorge in places less than 80 feet wide. In the summer the flow may be less than 10 thousand gallons a second, but, during floods, the flow commonly reaches a million gallons a second. The average flow is 82,500 gallons of water pouring over the falls every second and in a year, more than two and a half trillion gallons—enough water to flood the entire District of Columbia to a depth of 180 feet.

During two periods of recent flooding—once in 1936 and again in 1942—water covered the towpath of the C. and O. (Chesapeake and Ohio) Canal on the Maryland side of the river and also spread across the Visitor Center, parking lots, and maintenance building on the Virginia side. Floods of this size are most unusual, but about once every 2 years flood waters rise to the brink of the gorge at the falls, covering the overlooks on the Maryland side and reaching to within 15 or 20 feet of the trails along the Virginia cliffs. There have been 18 such floods since 1930.

Today Great Falls of the Potomac, Mather Gorge, and the surrounding scenic area administered by the National Park Service provide splendid opportunities for outdoor recreation. But perhaps most important of all, the area serves as a unique outdoor laboratory for nature studies and is a place to reflect on the impact of man on his natural environment.

Such studies require an understanding of the landscape—the geologic story concealed behind the scenery. This booklet presents a brief account of the geology of Great Falls, summarizing what is known of the events that formed the rocks and shaped the land.