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.
Entrapments: Debris such as fallen trees and tangles of fishing line can trap waders' feet, allowing fast currents to topple them. Ngo Tekwe Forchick of Takoma Park disappeared and drowned June 24 at Purple Horse Beach, a temptingly tame-looking spot with a strong eddy line just off the Billy Goat Trail.
Potholes: The riverbed is pocked with cracks, canyons and "potholes" drilled millennia ago by relentlessly churning water and rocks. Some potholes are 70 feet deep and large enough to swallow a car.
Cliffs: Four-story cliffs over calm-looking water tempt divers who don't understand the uneven terrain and erratic currents. The body of Army Spec. Vincent Crapps was found July 1, two days after he dived off a cliff called Cow's Hoof on the Virginia side.