It's a blustery April afternoon on the Colorado River, deep inside Grand Canyon National Park, and from his seat in the stern of a handsome little boat called the Sequoia, Martin Litton has just taken note of an intriguing development. Normally, the river down here is restless and kinetic, sluicing along with a muscular roll of its shoulders. At the moment, though, the current has mysteriously disappeared, and the water's surface has taken on the heavy, sullen stillness of a polished green gemstone.

Litton strokes his beard thoughtfully, then casts a glance behind the Sequoia to the boat I'm in, a yellow supply raft manned by a nervous-looking 57-year-old named John Blaustein. J. B., as everyone calls Blaustein, is sporting hipster shades and a cocoa-colored cowboy hat.

"You know, J. B.," Litton calls out, "when things get all calm like this, it means the river's backed up by something. And in this case, what it's backed up by is an absolutely terrifying pile of boulders called Dubendorff."

"Jesus, we're coming up on Dubendorff?" Blaustein yells back. A former Grand Canyon guide, J. B. now lives the agreeable life of a Berkeley-based commercial photographer, but once every summer he allows himself to be dragged down the river again. During these ordeals, he spends half his time pretending to complain about absurdly minor discomforts, the other half awash in a lather of angst over rapids like the one we're about to enter.

"I'm afraid it is," replies Litton, who's clad in a straw hat, an indigo shirt, and black suspenders—an ensemble that makes him look like an Amish farmer gone to sea. "This is a terrifying place, J. B. Absolutely terrifying."

"Absolutely terrifying" is Litton's favorite expression, a phrase he invokes several times an hour to describe everything from shifting weather patterns to the possibility that the six liters of Sheep Dip Scotch stowed in his hatch might run dry before the conclusion of this 280-mile odyssey through the rapids of the Grand Canyon.

There are 80 of these named rapids, a dozen of which serve up some of the biggest whitewater in North America. And though Litton and Blaustein know that Dubendorff, which marks the midpoint of most canyon trips, doesn't rank among the worst, it's not to be taken for granted. The run is a maelstrom of huge waves and sharp pour-overs that sound like the afterburners of an F-16. In the brief cushion of tranquillity before all hell breaks loose, Litton has a final thought to share.

"J. B.!" he barks.