Gauging Bolivia's gorges.

• Author: Buck, Daniel
• Position: Exploration of Rio Susisa Canyon

Bolivia may be home to the deepest gorge in the world. Then again, maybe not. Geology is a hard science, but canyoneering is more flexible. A relatively new endeavor, a sport more than a science, canyoneering is the conquest and exploration of the earth's chasms. Think of mountain climbing in reverse. Befitting a new game, the ground rules are still being hashed out.

In the center of one recent dispute is U.S. explorer Richard D. Fisher, who claims to have identified the deepest gorge in the world, the Rio Susisa Canyon, northeast of La Paz. The canyon, on the northern flank of Illimani in the Cordillera Real, plunges 5,084 feet from top to bottom and stretches 12,300 feet from rim to rim. (Fisher pulled the numbers off the contour lines on a topographical map rather than performing on-site calculations.) A leading canyoneer and director of Wilderness Research Expeditions in Tucson, Arizona, Fisher found the Susisa during a 1997 trip to Bolivia.

Distinguishing among the world's canyons, gorges, and valleys is no easy task. Nothing is set in stone. "As far as I am concerned," Fisher declares, "canyon and gorge are interchangeable terms." He says that he makes distinctions "purely on the basis of aesthetics," adding that "canyoneering is not definitional." Fisher believes that basic appearances are what count: A v-shape is a gorge, a stair-step is a canyon, and a u-shape is a valley.

Scientists find fault with that approach. The American Geological Institute's Glossary of Geology defines a gorge as "a narrow, deep valley with nearly vertical rocky walls enclosed by mountains." A canyon is similar, but larger--that is, deep but wider and longer. Valleys are usually very wide and shallower, with more gentle slopes, the result of glacial erosion or meandering rivers.

Some experts, such as Keith Echelmeyer, of the Geophysical Institute at the University of Alaska at Fairbanks, hold that a gorge is generally deeper than it is wide and a canyon wider than it is deep. The Great Gorge on Alaska's Mount McKinley, which Echelmeyer has studied extensively, is less than a mile wide and more than nine thousand feet deep, although the bottom third is...