http://www.washingtonpost.com/wp-srv/special/local/the-perils-of-great-falls/
THE RIVER CAN KILL — STAY OUT. That blunt warning greets visitors at Great Falls Park because subtlety hasn't deterred people from illegally wading, swimming and diving into this treacherous slice of the Potomac.
Since 2001, 27 people have died in river accidents in the area, including three since June. Few wore life jackets.
The death toll is low in the raging falls, because the danger is obvious and few people venture there. More often, the river's victims are people who came to hike, fish or swim and who disappeared after entering tame-looking water downstream.
The geology is complex, but in a nutshell, 200 million years of the river's flow barely eroded some of the bedrock. Waves of sediment from melting glaciers in western Pennsylvania exposed the same kinds of jagged cracks, outcroppings and rocks under the water as you see above it.
Water rushing through this obstacle course creates roiling underwater currents in even the calmest-looking places.
"People think, 'If this place was dangerous, they wouldn't let me be down here,' " said Lawrence Mullin, a member of the white-water rescue team from Fairfax County.
But people do get hurt in the park, and more than half the injuries that occur in the river are fatal, according to a 2011 study by the National Park Service.
This graphic looks at three types of areas, but most of the hazards listed can occur anywhere in the roughly 15-mile Potomac River Gorge, according to the scientists, kayakers, police and National Park Service officials who lent their expertise to this project.
FASTER THAN IT LOOKS
Narrower stretches such as this near the Rocky Islands may appear easy to cross, but currents quickly capture swimmers, waders and people who slip and fall into the water.
ILLUSION OF CALM
In a straight crack in the bedrock called Mather Gorge, gravity keeps the surface placid while currents below can be as strong and chaotic as white-water sections.
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