FOUR YEARS AGO, at age 38, I wandered through the doors of Grand Canyon Dories, an outfitter in Flagstaff, Arizona, that runs commercial expeditions on the Colorado River, and found myself staring at a tiny navy of wood-and-fiberglass rowboats. There were only about a dozen of these sleek, flat-bottomed craft inside the boathouse on that morning in March 2004. They were all painted in bold colors, and several featured flat transoms inscribed with hand-drawn scenes from the river: a bighorn sheep, a cluster of columbines, a tree frog. Each boat was graced with the simplest and loveliest lines I'd ever seen.

At the time, I had no idea that dory boats, which had been used for centuries by cod fishermen on the gale-racked combers off the Grand Banks of Newfoundland, had become legendary on the Colorado, where they are renowned for their speed and elegance. What I did know was that I was entranced. And in an impulse that defied logic and common sense, I decided—right then—that I was going to quit my job and somehow find a way to follow those boats into the water-haunted world at the bottom of the Grand Canyon.

Plenty of middle-aged men flirt with delusional schemes like this—and almost all of them wind up abandoning their fantasies once they come to their senses. For better and worse, I followed through on mine, even after I found out that becoming a guide involves a multiyear, unpaid apprenticeship in the exciting field of latrine management.

I didn't care. In fact, over the next four summers, I volunteered for extra poo-boat duty to compensate for my lack of skill in crucial areas like, say, rowing. In the course of 14 trips through the canyon, I have transported more than 7,800 pounds of excrement over a total distance of 3,400 river miles—roughly equivalent to rowing an inflatable septic tank from Tijuana, Mexico, to Point Barrow, Alaska. The price I've paid for this dubious distinction has been both humbling and steep. At 43, I find myself with no wife, no kids, no dog, and virtually no bank account. Last year, according to a nationwide survey of incomes across the U.S., I made less money than a part-time doughnut fryer in Maryland and a hospital clown in New York.

For the most part, it's been worth it—I wouldn't trade my Grand Canyon experiences for anything. My other consolation is that poo-boating, gross though it may be, is important work. Every summer, about 25,000 people opt to ride motorboats, oar rafts, or dories down all or part of the river corridor through the canyon, a route that stretches 277 miles from the eastern head, at Lees Ferry, to the western terminus, on the shores of Lake Mead. These trips usually take between one and three weeks, and because the average person will produce an ounce of excrement for every 12 pounds of body weight each and every day, the river-running industry generates more than 100,000 pounds of human waste per year. Inside a narrow canyon that receives less than nine inches of annual rainfall and endures summer daytime highs in excess of 110 degrees, it doesn't take much to imagine how ugly things could get if waste weren't handled with extreme care.

Up at park HQ, they take this business seriously indeed. The River Permit Office, on the South Rim, distributes a 45-minute video with detailed instructions on how river trips must containerize human waste, cart it out in watertight boxes, and dispose of it at a sewage-treatment plant. No trip is allowed on the river without proper education for crew and passengers and a written commitment to pack it out. It's a key component—perhaps the key component—of a complex set of rules that keep the crown jewel of the U.S. national park system as pristine as possible.

It's also something most people don't like to talk about.

If you sign on for a commercial Grand Canyon river trip, you'll learn about all sorts of fascinating things, from the Colorado's maximum depth (85 feet) and minimum width (76 feet) to the number of rock layers exposed along its length (26). The subject you won't hear discussed is the one I'm most attuned to: poop handling, and the headaches and disasters it can entail. For example, you probably won't hear about the two guides who were driving back from a river trip when their poo cans fell off the back of their truck and burst open in the middle of the road, creating a fecal slick that a bicycle tourist wiped out on. Or the unlucky boatman who had a leaky toilet transform his rubber raft into a tub of raw sewage. Or the hapless guide who tripped while setting up the lavatory, plunged his arm into the poo box, and compounded his problems by vomiting all over himself.

Mishaps like these—which are the exception, not the rule, on most whitewater trips—underscore an unsettling fact that most commercial passengers never have to confront. To wit: A river runs through your Grand Canyon fantasies, and it's not clean and sparkling. If I do my job right, you won't have to dwell on that miserable reality. In a way, I'm like Quasimodo with an oar and a toilet brush. I'm not easy to look at, but things work a whole lot better when I'm around.