IT'S A FIERY JUNE MORNING at Grapevine Camp, a spit of sand tucked along the banks of the Colorado River, deep inside the stone walls that frame the sub-basement of the Grand Canyon. From the surface of the river, the walls soar upward for more than a vertical mile, exposing geology that extends 17 million centuries into the past. During that span, the oceans have swollen and receded a dozen times, the continents have slammed together and cracked apart again, and a chain of mountains higher than the Rockies has been heaved into the sky and reduced to gravel.

Philosophically speaking, this is some heavy shit. Heavy enough to make a man perched on the bow of a humble raft at the edge of Grapevine—a man now staring at that staggering immensity of stone—scratch his head and wonder what it all might suggest about his own place in the universe.

But that doesn't last long. Any confusion about where I fit into the cosmos is vaporized by the arrival of a hefty steel box that two of the guides on this 19-day river trip are slinging onto the aluminum deck of my raft with a rude, clattering ka-thunk.

"Heads up, there, my friend," warns Bill "Bronco" Bruchak, a boatman who's built like the beer truck he used to drive in Pennsylvania. "Don't pull a muscle when you lift this thing."

"Yep," chimes in Mike "Milty" Davis, a small, cheerful guy with mischievous eyes and a snowy white beard. "That is one enormous box of poop."

I seize the handles, heft the cargo, and stagger toward the tight space between the stanchions that cradle my 12-foot-long, fiberglass-reinforced oars. Two identical canisters are already anchored on both sides of the footwell, which is where I sit when I row this barge. The top of each can is emblazoned with a strip of red electrical tape labeled FULL!!!

As I start lashing down the new can, I glance over at Monte Tillinghast, who's piloting the second baggage boat on this trip, a kitchen raft that's tied up next to mine.

"Son, you do have a load there," observes Tillinghast, whose cowboy hat and surfer shorts make him look like a maritime version of the Marlboro Man. "You know, you might want to think about girth-hitching another strap onto that there—"

Monte's suggestion, undoubtedly helpful and possibly of critical importance in how my day is about to unfold, is cut off when Andre Potochnik, the alpha boatman leading this trip, yells out, "Dories ... ho!"

This sends Monte and me scrambling to complete our rigging and fall in behind the four dories, which, one by one, are now threading into the main current. As the last of these craft glides past my barge, the final member of our crew—a slim, deeply tanned woman named Billie Prosser—flashes a smile. "Hey, Kevin," she says sweetly. "Thanks again for doing what you do."

Her four passengers nod gamely, but their smiles are a frozen mix of polite gratitude and mild repugnance. After nearly a week on the river, these folks know exactly what's in the boxes on my boat. After all, they put it there. And although they're deeply obliged for my services, they're still trying to get a handle on who I am, what my job entails, and why in the world I agree to do it. It's a look that says, Someone give this poor idiot some career counseling. Immediately.