| Great Falls | |
|---|---|
 The Great Falls as seen fromNorthern Virginia | |
 Interactive map of Great Falls | |
| Location | Montgomery County, Maryland andFairfax County, Virginia, U.S. |
| Coordinates | 38°59′51″N77°15′09″W / 38.99750°N 77.25250°W /38.99750; -77.25250 |
| Type | Steep Tiered Cascades |
| Elevation | 129 ft (39 m) |
| Total height | 47 ft (14 m)[1] |
| Average width | 350 ft (110 m)[1] |
| Run | 550 ft (170 m)[1] |
| Watercourse | Potomac River |
| Average flow rate | 16,750 cu ft/s (474 m3/s) (est.)[1] |
Great Falls is a series of rapids and waterfalls on thePotomac River in the state ofMaryland 14 miles (23 km) upstream fromWashington, D.C. As the border between the states of Maryland andVirginia generally follows the low-water line on the Virginia side,[citation needed] the river and falls at Great Falls lie withinMontgomery County, Maryland, which abutsFairfax County, Virginia on the far shore.
Great Falls Park, managed as part ofGeorge Washington Memorial Parkway, is on the southern banks inVirginia, andChesapeake and Ohio Canal National Historical Park parkland is along the northern banks of the river inMaryland. Both are operated by theNational Park Service.
The Great Falls area is popular for outdoor activities such askayaking,whitewater rafting,rock climbing, andhiking. TheBilly Goat Trail onBear Island, andOlmsted Island, both accessible from Maryland, offer scenic views of the Great Falls. There also are overlook points on the Virginia side.
Great Falls andLittle Falls (about 5 miles downstream) are named in contradistinction to one another.
The river cascades over a series of 20-foot (6.1 m) falls, dropping a total of 76 feet (23 m) in elevation over a distance of less than 1 mile (1.6 km).[2]
The Potomac narrows as it passes over the falls and throughMather Gorge. Heavy rain or snow in the watershed upstream causes white-water floods which entirely submerge the rocks and even threaten the adjacent park visitor center (built on stilts for this reason). A pillar at the Virginia overlook, well above the river, marks the level reached during the1936 Northeastern United States Floods.
The rocks of the falls are about 750 million years old, dating to the latePrecambrian era.[3] They are composed of a resistantmetamorphizedschist,gneisses, metagraywackes, and metaconglomerates.[3] The Falls themselves formed during theLast Glacial Period, about 35,000 years ago, when sea level most recently dropped, causing the Potomac todowncut its valley.[3]
Multiple canal systems were built around Great Falls. The remains of thePatowmack Canal, built in the 18th century, can be found on the Virginia side. The canal was commissioned byGeorge Washington and consisted of a system of five locks to allow barges to avoid the falls.[4] Later on, theChesapeake and Ohio Canal was built in the early 19th century on the Maryland side of the falls. It ultimately connected the Potomac tidewater withCumberland, Maryland.[5] The Chesapeake and Ohio Canal also used the Great Falls as a feeder (now abandoned) to supply water for its own use.
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