(FORTUNE Magazine) – Floating quietly, I'm enjoying a final moment of peace before the madness begins. I'm sprawled out on what is really just a big kickboard, and it seems woefully inadequate for navigating the ferocious rapids downstream. Suddenly I'm grateful for all the protective gear--padded body suit, life jacket, crash helmet.

"There'll be some big waves!" shouts Eric Adams, one of the guides at Go Big Whitewater. "So hold on to your board!"

Clutch the sucker with all your might is more like it. For the next five hours and 18 miles, five of us follow three guides on a white-knuckle trip through (and under) rapids with increasingly scary names: Rock & Roll, Son of Satan, Meat Grinder.

Riverboarding, says Adams, is "Boogie-boarding on steroids." Basically, you lie on a four-foot slab of hard foam, grip the handles, kick with flippered feet--and in our case, shoot ten-foot-tall Class III rapids.

The father of U.S. riverboarding is Bob Carlson. In 1975 he dropped out of a Berkeley doctoral math program to start a rafting-gear company. At that time white-water rescue classes were taught with Boogie boards--but the thin, slick boards sank in heavily aerated white water and tended to shoot out from under riders. So Carlson made them bigger, easier to hold, and more flexible. He began selling his boards in 1985, mostly to rescue teams, and soon after, river guides started using the boards for thrills.

Our trip is led by Adams, Kacy Fitch, and Dan Ford, all of whom have been riverboarding for at least six years (it was Ford who started Go Big Whitewater, in 1997). Adams and Fitch zip around on riverboards, giving us tips on how to set up for each batch of rapids. Ford mans the raft stocked with food, water, and first aid--and provides comic relief. He tosses out beers from a case he found floating in the river to kayakers drifting by.

The highlight is rodeo riding. Riverboarders use parts of the river called hydraulics, or holes, to ride in one place. Hydraulics are formed when the river pours over an obstacle and drops toward the bottom--causing the water to tumble back on itself and spin as if in a washing machine. That creates a powerful vortex that can hold riverboarders in position while facing upstream. Kayakers use holes for rodeo tournaments and earn points for performing tricks.

Around lunchtime we approach a hydraulic called, yes, Rodeo. The first rider up is Dan, a motorcycle mechanic who's been riverboarding for eight years. He splashes down on his board right in the hole--riding it for a while and then rising to his knees, stretching out his arms as if he's surfing. We cheer. John, a fiftysomething Seattle businessman and first-timer, tries to kick his way into the hydraulic, but the current is too strong, and he gets swept downstream. Now it's my turn. I make it into the hole, struggling to hang on as my board bucks like a bronco (so that's why ...). I feel as if I'm going 60 miles per hour as I surf what seems like a tsunami coming right at me. In a word, it's exhilarating. But then I accidentally let the nose of my board dip below the wave. Big mistake. Within seconds I'm 20 yards downstream, wondering which way is up. Ford pulls me ashore. "Nice ride," he says, slapping me on the back. I cough up half the river.

The sport's junkies claim it's safer than rafting and kayaking, and by the end of the trip I begin to believe them. Unlike rafting, you won't get tossed into the water unexpectedly (because you're already there). And unlike kayaking, you're not doing any rolls where you can crack your head against a rock, tear your shoulders, or get stuck upside down and drown. Indeed, Ford says there's never been an accident on one of his trips--"just sore muscles."

"You'll see a big wall of white," says Adams, about a massive, frothy wave in the last set of rapids. It has an ominous name: Suffocator. We fan out across the river, and with our fancy gear we look like Navy SEALs on a mission. The current speeds up, and soon we're crashing through the rapids. Suffocator gets bigger and louder as we approach it. Bring it on.