BUENA VISTA — Whitewater stand-up paddleboarding is blowing up, with Colorado emerging as the proving grounds for the suddenly everywhere sport.

After a few years of slow and steady growth, stand-up paddlers now clog eddies alongside kayakers in Colorado's flowing rivers. Competitions this season — at the Golden Games and Buena Vista's PaddleFest — saw the number of racers more than triple from last year.

"Now I get to tell everyone 'I told you so,' " said Ken Hoeve, a Gypsum stand-up athlete who guides, competes and just started a SUP yoga gig called SUPeaceful, which hosts river yoga sessions on paddleboards and supports local efforts to curb domestic violence.

Stand-up paddling is the snowboarding of whitewater, an upstart sport elbowing for legitimacy among kayaks and rafts. Born on the beach, the standing strokers have moved inland, surfing river waves and racing downriver. Colorado is home to two paddleboard companies — Badfish in Salida and Hala Gear in Steamboat Springs — that design boards for river running and river surfing.

"The equipment is finally being built to be paddled inland with boards that make people successful in the river right off the bat," said Peter Hall, whose 2-year-old inflatable Hala river boards are quickly gaining a foothold in the nascent sport dominated by ocean board makers. "Now when people get started, they are hooked."

Retailers across the state are reporting brisk SUP business with river-inspired inflatables leading the charge.

"I have been hearing more and more customers say they are interested in moving water this year," said John Bridenbaugh, whose Altitude Paddleboards shop in Littleton sells only paddleboards and SUP gear.

Bridenbaugh said last season's weak flows pushed a lot of kayakers into SUP.

"There are plenty of adrenaline junkies out there looking for the next plateau, and there is an uptick in interest from experienced paddlers who see what a blast we are having and want to drink the Kool-Aid," Bridenbaugh said.

"The cat is definitely out of the bag," said Earl Richmond, whose Colorado Kayak Supply in Buena Vista hosted the busiest PaddleFest in years, thanks in large part to the influx of SUP events and contests.

Saturday night's upriver-downriver SUP race featured the biggest names in the sport, with pioneers of whitewater SUP — like Aspen legend Charlie MacArthur and big-water paddler Dan Gavere — battling in a course that wound up and down Buena Vista's play park. That park, like many across Colorado, now features SUP-specific waves that allow for traditional board surfing on the river.

Richmond said "there's a lot more traction in whitewater SUP than there is in whitewater kayaking right now."

"People are more comfortable doing it. It's more accessible," he said. "It seems like everyone wants to try it out. Designers are coming around with more river-centric boards and the hype is high for SUP right now."

Miles Harvey, the 11-year-old Salida paddler who won the junior race Saturday at PaddleFest, thinks whitewater stand-up could eclipse kayaking.

"It's definitely easier for people to learn. It's easier for them mentally to get into it because they don't have to be worried about flipping upside down," said Harvey, whose dad, Mike, co-founded river boardmaker Badfish. "And it's so fun."

Jason Blevins: 303-954-1374, jblevins@denverpost.com or twitter.com/jasontblevins