Raised in Hawaii and Southern California, Charlie MacArthur is no stranger to surfing. He began riding waves at age 7, shortly after his father James moved the family to Oahu to take the role of Dan 'Danno' Williams on a new television show called Hawaii Five-O. As a young man he moved to Aspen, Colorado, and began seeking his liquid fix on Rocky Mountain whitewater. Eventually he became one of the region's best kayakers, notching several notable first descents and opening the Aspen Kayak Academy. But the surf always beckoned, and when standup paddleboards came onto the scene, MacArthur immediately began exploring their potential on rapids and standing river waves. Now 50, he's a raving advocate for river SUP. He opened the Aspen SUP School, spearheads the Whitewater SUP Championships, rides and designs boards for C4 Waterman, and may be the only person alive who can perform a kayak-style roll on a standup paddleboard. With a growing number of SUP first descents to his credit, the man everyone knows as C-Mac is shredding river waves and stereotypes as relentlessly as his dad booked TV bad guys. – Eugene Buchanan

Surfers don't know a thing about rolling. If you can't punch through an oncoming ocean wave, you have to turtle. I knew I was going to start doing rivers, so I started working on it. Basically, it's a sweep roll. You crunch up in a ball next to your board, put your surface leg across it and then pull your other leg under the water toward you while sweeping your paddle. You end up laying on your back and then you just do a sit-up and stand up.

It's a fun, cool thing to do. But it remains to be seen if it's super-functional or just a novelty. You have to have foot straps to do it. You can get out of them quickly, but you have to be careful about tweaking your ankle.

Last year the judges at the national Whitewater SUP Championships made a mistake. They placed me in front of Dan [Gavere], who beat me in downriver, surfing and slalom. So they went back and changed it. Dan won fair and square.

There was a lot of carnage at this year's SUP-cross event. It was the first time we ever held it. We ran down a ramp, did a Le Mans start, and then jumped on our boards and peeled out into the river. It had everything you'd expect in a 'cross event, with tempers flaring and people interfering with each other. Everyone got all fired up.

People stepped it up at this year's championships. A couple of guys pulled off 3's [360-degree spins) on the wave. I got second in the downriver and third in the 'cross, but didn't do so well in the surf event. I was standing in the water helping push the women onto the wave and froze my ass off. So I was all stiff when my heat came up. But there were some hot girls, so it was great.

For the C-Mac, I wanted to design an all-around board that could surf river waves well, cruise lakes and surf the ocean. But I also wanted it light and thick enough to work on a river. I think we succeeded. There's also an inflatable version out now.

There's a lot of crossover from kayaking when you take a standup to the river. The cool thing is going from a kayak with no fins to a board with deep ones. It lets you do surf-style moves with the tail. If you don't want the fins, you can turn your board around. Your stance is also unlimited, and so is what you can do with the paddle. The possibilities are endless.

Between the nationals and Teva Mountain Games, river SUPing is getting more exposure than ever. It should continue to grow. This year's nationals had four times as many spectators as the year before.

I'm not a great surfer. It's great to combine what I know about kayaking to standup paddling. It makes it easier. SUP river races definitely favor kayakers.

I'm proud that my dad played Danno on Hawaii Five-O. They're making a new show now also. James Caan's son will be playing Danno. McGarrett and Danno are going to be more like Starsky and Hutch than they were in the original.

I've taken my dad kayaking a few times. He also enjoys watching me standup paddle.

I'm rolling the Aspen SUP School into the Aspen Kayak Academy. It's attracting a whole new demographic. People who can't see themselves getting into a kayak are coming out of the woodwork to take SUP lessons.

All we've done is transplant it from the ocean inland.

There's a lot of animosity among ocean surfers. It's much less of an issue on the river. People hang out and appreciate each other, rather than just tolerating one another. The camaraderie among river-runners blows surfers away.