Maryland Department of Natural Resources

The Great Falls of the Potomac, Montgomery County

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The Great Falls of the Potomac River are located west of Washington, DC in Great Falls Park. This series of cascading falls has developed on the Potomac River in rocks formerly known as the Wissahickon formation. The Wissahickon consists of resistant metamorphic rocks including schists, gneisses, metagraywackes and metaconglomerates. These rocks date from the late Precambrian and are about 750 million years old. The Falls formed when sea level dropped during the last Ice Age, causing the Potomac to downcut its valley.

The Great Falls
The Great Falls

The Great Falls are actually a series of cascades and rapids on a two-thirds of a mile stretch of the Potomac. The river drops about 76 feet over this distance. None of the individual falls exceeds a 20 foot drop.

Because the Falls presented a major obstacle to 18th century boat traffic, George Washington proposed the construction of the Patowmack Canal at Great Falls in 1784. The project required 16 years to complete.

For more information on waterfalls refer to FactSheet 9: Maryland's Five Highest Waterfalls and Mountains.