"So, is the surf up today, boys?" the old cowboy asks as he ambles up to the stoop of Mark's In & Out Drive-in, where we're feasting on cheeseburgers, onion rings, and huckleberry milkshakes. He's wearing a 10-gallon hat, starched jeans and a sun-creased smile.

I swallow a huge bite of burger, thinking how best to answer.

"Well, sir, we hope so."
"So are those surfboards?" he continues, gesturing to the quiver of brightly colored standup boards strapped to the top of Walker's truck.
"Sort of. They're standup paddleboards," I explain. "We're going to paddle them down the Yellowstone."
"Through the rapids at Yankee Jim too?" he asks.

With that he gives us a puzzled, you-ain't-from-around-here look, then smiles again and wishes us well.

The truth is, none of us are from around here. I'm from coastal North Carolina. Walker Ferguson is from Boulder, Colorado, while Becky Williams and Mike Harrelson both hail from Bozeman, Montana. She was raised on an Iowa farm, and Mike grew up on the beaches of Virginia, Maui and Ventura, California.

We came together on this fine late August day in Livingston, Montana, an old railroad town and gateway to Yellowstone National Park, to embark on a five-day, 65-mile float down the Yellowstone, the longest free-flowing river in the Lower 48.

The trip was hatched when I first met Mike at a party a few weeks earlier. He'd read a couple of my surf expedition articles over the years, and as a displaced inland surfer, he was jonesing to talk waves. Soon we were swapping surf stories. Turns out, he works for an ad agency that promotes Montana tourism, which plays right into his never-ending mission to combine work and play. In less than 10 minutes he concocted this SUP expedition.

I was immediately intrigued. Though I regularly paddle my SUP around the Intracoastal Waterway near my home, I'd never before paddled one down a river—or a whitewater rapid for that matter. Neither had Mike. That's why he recruited Walker, a stoked-out whitewater board-rider who happens to be the son of one of Mike's oldest friends.

The plan was for the three of us to paddle our boards downriver, running rapids, looking for waves to surf and camping on sandbars and beaches along the way. Though we would be the first people ever to paddle this stretch of the Yellowstone on SUPs, Mike had no intentions of roughing it. He invited his coworker, the lovely Becky, to follow us in her new inflatable raft loaded down with all our gear, food and beer.

We put on the river about eight miles north of Yellowstone National Park and ride the current northwest through the golden prairies and gentle scrub-brush hills of Gallatin National Forest. Though U.S. Highway 89 parallels the river closely in places, down here on the river it's quiet. And civilization seems worlds away.

Our first camp is a tiny teardrop-shaped sandbar in the middle of the Yellowstone, where we bed down under the stars in a patch of soft sand. In the morning I wake to the smell of Mike brewing coffee. Becky is manning a can of Batter Blaster, whipping up stacks of blueberry pancakes.

Our route mirrors that of the great explorers Meriwether Lewis and William Clark. Those characters and their spelling-challenged journals show up in every travel story and tourist brochure ever printed about Montana, but I feel I've earned the right to reference them. We're not exactly breaking new ground, but we're the first to pass this way on standup paddleboards. It's easy to convince myself that little has changed since the Corps of Discovery first traveled this stretch of river in 1805, and like them, we're anxious about what lies ahead. In particular, we don't know what to expect from Yankee Jim Canyon, a whitewater section that Capt. Lewis described as "extreemly rappid."

If Walker is nervous, he's not showing it. The ultra-fit 27-year-old, a former junior world mountain bike champion, is playfully bopping down the river, sliding into eddies (the stagnant water behind rocks) and then planting the tail and peeling out back into the current. Mike, lean and fit at 51, is doing his best to keep up. After many years of surfing and paddling decked canoes he's at home on the river, and seems to relish the challenge of matching Walker stroke for stroke, eddy for eddy. Becky, 30, is trailing lazily behind, soaking in the morning sun and obviously stoked she's not stuck behind a desk for the week. She is that most glamorous brand of mountain girl, a pretty tomboy, and her mellow demeanor makes her seem confident on the water. In reality though, she's only been paddling for a few months, and this is her first overnight river trip. "I'm a non-swimmer," she admits matter-of-factly. "I just started water sports a couple years ago."

We're more than a dozen miles into our journey when we enter Yankee Jim Canyon, named for the pioneer, James George, who in the 1870s gained squatter's rights to the only road through the canyon, and the main route into Yellowstone Park. There he built a cabin and tollbooth, levying a $1 toll for all on horseback and $2.50 for wagons. Yankee Jim was effectively a troll under a bridge. A toll troll.

Just above the first drop, a rapid called Boat Eater that rates III out of V on the international scale of river difficulty, we pull over to remove the long center fins from our boards. With the two smaller side-bite fins still attached, Walker says we'll have some directional control but less danger of knocking into rocks or getting thrown off-course by squirrely crosscurrents. We also strap leashes to our boards and attach the free end to quick-release rescue belts on our life jackets. That way we won't risk losing our boards downriver after the inevitable wipeouts, and if a leash gets dangerously hung on a rock, we can easily bail.

I'm the first to enter Boat Eater. Here the canyon narrows, creating a large breaking wave in the center of the river, followed by a rowdy chain of standing waves. I add a few strokes to set up my angle before I'm slurped down the chute and into the rapid. It's possible to skirt along the side, but I decide to blast through the meat of it. I paddle hard and punch up and over the first wave. The nose of my board rockets off the back, then, like a teeter-totter, tips back down and buries its nose deep in the trough of the next wave. My board stops dead. I keep going. The cold water makes me gasp as I'm swept through the turbulent waves. I rush to collect my paddle and board so I can make it to shore in time to photograph my friends and their runs.

According to the whitewater rafting outfitters who run trips on the Yellowstone, Boat Eater flips more rafts than any other rapid on the river. And I can attest, it's also damn fine at dismounting SUPers. Becky makes it through relatively smoothly, but both Mike and Walker are served a spanking similar to mine.

I quickly realize that paddling whitewater on a SUP is extremely challenging. Unlike the ocean where waves are coming at you from one general direction and you're dealing with just a couple different currents, in whitewater the waves come from all directions. Countless unseen currents will send your board this way and that. Your muscles are rapid firing, constantly making the tiny corrections necessary to keep you upright and on the board.

"When you're going through rapids, the trick seems to be having your paddle out for support, like a tripod," deduces Mike as we regroup after the carnage at Boat Eater.

Honestly, I'm humbled and frustrated by how difficult it is to stay upright through an entire rapid. I figured I'd be well suited for whitewater SUPing. I've been surfing and kayaking much of my life, and spent five summers as a whitewater rafting guide on Tennessee's Ocoee River. I even worked for several years as a gondolier in San Diego Bay (No, I can't sing; Yes, I've paddled in Venice). But I'll be damned if I don't fall in every single rapid on this three-plus-mile stretch: Big Rock, Box Car, even the drops that don't have names.

Still, the constant challenge makes it insanely fun. We spend most of the day and the bulk of our energy carrying our boards back up the riverbank to paddle the rapids over and over again, trying to catch waves and ferrying across the stiff current.

"There are a lot of these older paddlers with families that aren't into pushing the limits in a kayak in Class V whitewater anymore," Walker explains as we feast on a late lunch in a sun-drenched spot just below Yankee Jim. "But they have a blast coming back to Class III on a SUP. Because it's so hard, and because there's not so many consequences."

I nod my head in quiet affirmation. As a former raft guide and a 37-year-old father of two young boys, I can totally relate.

"It was really fun trying to surf some of those open river waves in there," says Mike, an otherwise landlocked surfer. "You can catch it for a second but that's it. The speed of the water going under the board is pretty nuts."

Lunch is quite good too. Fully loaded bagel sandwiches with fresh deli meat, tomato, avocado, plus chips, salsa and ice-cold Kokanee beer. Life is good.

Refueled and still buzzing about our successful descent through Yankee Jim, we paddle all afternoon, putting down about 16 more miles. Just before it grows too dark to continue, we finally find the perfect campsite not far from Emigrant Creek. It's a small pocket beach on the left bank with the softest sand imaginable. Reading Capt. Lewis' journal entry from April 26, 1805, I chuckle at the similarities in our experience and theirs: "Having arrived at this long wished for spot," Lewis wrote, "we ordered a dram to be issued to each person; this soon produced the fiddle, and they spent the evening with much hilarity, singing & dancing, and seemed as perfectly to forget their past toils, as they appeared regardless of those to come."

We crack open the Kokanees again and dive into the best damn burrito feast in SUP river-trip history. Then we produce drams of our own and eventually launch into a little a cappella karaoke.

"It's about time you guys started drinking some of that whiskey," says Mike.
"But who's gonna drive tonight," I joke.
"I'm gonna drive," laughs Walker.

Ironically, he does, even though his pickup is waiting some 40 miles downstream at our takeout. Around midnight, Walker decides it's time for a standup session. Before we can dissuade him, he hops to his feet, begins paddling, and promptly falls ass over teakettle into the black water. We laugh hysterically and I throw a life jacket to his feet. With that, Walker disappears into the night. As he ferries his board back and forth in the stiff current, the only thing we can see is the tiny beam of his headlamp hovering above the Yellowstone.

Though this trip is all about the paddling, it turns out the camping is at least half the fun. Like car camping, there's no real limit to the sort of stuff you can bring along on a river trip. And thanks to Becky and her raft, we have a huge cooler full of better food than I eat at home, Knob Creek whiskey and cases of beer. There are lawn chairs and a stove, tents, sleeping bags, everything you could want. Each night we have campfires, and too many Kokanees, and then fall asleep under a canopy of stars.

Thanks to Montana laws, we can camp anywhere we please as long as it's below the river's high-water mark. And the choices are many. Each of the campsites we find look like something out of an REI commercial—beaches of fine black and gray sand bordered by riprap rocks and cobblestones. Just beyond the river's edge the beaches give way to lush green meadows full of purple thistles and huge stands of cottonwoods. There are mustard yellow prairie grasslands, hemmed in by steep granite peaks. On river-left, to our west, it's the jagged Gallatin Range and on river-right, the Absaroka-Beartooth Wilderness. The term Big Sky is often overused to describe Montana, but in this case it's pretty much right on.

Another beautiful thing about Montana and paddling the Yellowstone is the decided lack of humans. Most days we see more wildlife than we do people. Maybe it's a bald eagle soaring off with a freshly caught blue-ribbon trout in his talons, or a herd of pronghorns bounding alongside us on the riverbank, then disappearing in a flash of white tails.

In late August, the Yellowstone is swimming-pool clear. The water is so transparent that it's difficult to judge how deep the river is. Sometimes I catch myself just standing there, staring. I don't paddle. Instead I fixate on the smooth brown and tan river rocks as they whiz past below my feet. Most times I just let the stones blur into one and watch the river roll by. Occasionally I catch a glimpse of a startled cutthroat or rainbow bolting past the nose of my board. As long as the sun doesn't cast my shadow downriver, it seems I'm a total surprise to the fish.

In the same way we sneak up on the trout, our boards allow us to silently approach other wildlife. One afternoon we slip quietly up to a group of young white-tailed deer. We expect them to bolt immediately but instead they just stand there, ears peaked, eyeing us curiously. Moments later a huge white pelican touches down nearby with a rowdy splash.

"What the hell is he doing here?" I ask.
"Good fish here too, bro," laughs Mike. "This is inland sushi."

Good fish indeed. On day three, in a stretch between Emigrant Creek and a spot called Mallard's Rest, we encounter several groups of flyfishermen on drift boats and prams (a skiff with squared-off bow and stern). Though there are never more than a couple boats in sight at any one time, we're a little taken aback by the presence of other folks on the river. Thankfully, they're gone by sunset and the Yellowstone becomes all ours once again.

It isn't until late on day four as we approach Livingston, about 55 miles into the trip, that the river becomes relatively busy again. We pass folks paddling kayaks, rafts full of partying high school kids, a flotilla of inner-tubers cruising the rapids through town, and more flyfishermen.

I look ahead to see Walker and Mike as they slip around a crescent-shaped bend in the river. The guys are paddling side-by-side and the current seems to pull them effortlessly downriver, like they're riding a moving sidewalk. Like they're walking on water.

"What do you call that?" a bearded angler yells to me from the riverbank.
"Standup paddleboarding," I shout back.
"I guess so," he says. "Looks really fun."
"It is, sir. It really is."

Words by Mark Anders
Photographs by Brian Grossenbacher and Mark Anders