A dozen blue rafts bobbed lazily down a calm stretch of the Merced River just downstream from Yosemite National Park earlier this month.

The clear mountain water was tinged with a greenish hue as the mostly first-time rafters headed inescapably toward a wall of churning whitewater.

Twentysomething rafting guide Sean DeAvila with Columbia-based Zephyr River Expeditions called out commands to his novice crew as their 14-foot rubber boat swiftly approached the foaming rapids.

"Forward!" he shouted as the quartet of German tourists paddled furiously to crest the first big wave.

The boat bounced, shuttered and nearly flipped as it bounded off polished river rocks like an oversized pinball. Ice cold water doused the occupants as they plowed through standing waves and into deep, watery troughs.

"Don't stop yet, go, go, go!" He yelled again.

DeAvila's job was to shepherd the newly minted rafters through the seemingly impassable rapids, which carried menacing names like Road Rash, Dave's Dump Truck and Monster Mash. He guided them past sharp rocks, boat-flipping whirlpools and a succession of other hazards until they safely reached another patch of calm green water.

"I know this river like the back of my hand," he said as he steered the craft with the help of two long wooden oars at the boat's stern.

With the onset of warmer weather and rising river flows from snowmelt, rafting season has hit full swing, according to Bob Ferguson, who founded Zephyr Expeditions in 1973. His company takes roughly 10,000 people per year on the ride of their lives with whitewater rafting trips down the Tuolumne, Stanislaus, Kings, American and Merced rivers.

"A lot of our clients say it is the best thing they've ever done in their lives," he said.

Birgit Schuster was visiting California on a four-week tour with her parents, Karsten and Christa Schuster, and her wife, Marlese Badenhorst. The family jumped at the chance for a whitewater rafting adventure when they saw advertisements online.

"My favorite part was when wenearly capsized, actually," she said with a chuckle.

Los Angeles area production sound mixer Robert Kennedy was on his first whitewater rafting trip with his girlfriend Julie Therien, who is an experienced rafter.

"It was one huge wave after another," Kennedy said. "It was like up and down, up and down."

Rafting season will likely be shorter than normal on most California rivers due to less-than-expected snowfall in the Sierra Nevada, but water levels are excellent for the time being, he said. Rafting tours have been running nearly every day on the Merced and Stanislaus rivers for the past month.

The river water levels will fluctuate depending on the weather, Ferguson said. Warmer days will cause more snowmelt to flow down creeks and streams and will increase the water level. He said peak flow for unregulated rivers like the Merced will likely come later this month.

Tours of the more-advanced rapids on the Tuolumne began last week, and Ferguson said higher than normal water releases from Hetch Hetchy Reservoir will result in top-notch rafting conditions through Labor Day.

"The Tuolumne River will have a banner year," he said. "Dam releases will give us better water than normal."

Organized rafting trips cost from about $80 for half-day trips to $585 for three-day expeditions. The standard 14-foot boats carries six paddlers and one guide. Two-day trips on the Tuolumne River require more gear than single day or half day trips, so the rafters are usually followed by a cargo boat. Multi-day trips are comprised of about 20 rafters, Ferguson said.

While whitewater rafting is safe compared to a wide range of other extreme sports, it is not without risk. An average of six deaths per year occur on commercial whitewater rafting trips nationwide, according to accident statistics provided by the nonprofit group American Whitewater.

Professional whitewater outfits generally require first-time rafters to complete a brief training session during which a guide explains what to do in case the boat flips or a person falls into the water. In some instances, they could be required to complete a swim test before being allowed to hit the water.

The Forest Service requires guides to be trained in first aid, CPR and have more than 500 miles of river navigating experience.

Standard safety gear for rafters includes plastic helmets, life vests, splash jackets and wetsuits in cooler months.

Rafting season generally lasts from May to August in California and the western states, while on the East Coast the season extends from March to October.

Permits to raft California rivers are issued by either the state Bureau of Land Management or the U.S. Forest Service.

Marty McDonnell, owner of Sierra Mac River Trips in Groveland, has been leading rafting expeditions on the Tuolumne River since 1965.

He said rapids are generally placed in one of six categories in ascending order depending on their treacherousness and technical difficulty. A "Class 1" is the lowest category, while a "Class 6" is considered dangerous even for the most experienced river rafters.

He said dam releases into Cherry Creek on the upper portion of the Tuolumne River have resulted in Class 5 rapids, which are considered some of the most difficult river flows.

He said river conditions are more or less average this year, while last year saw near record-setting river flows that created prime conditions for whitewater rafting. On the Merced River, for example, 3,000 cubic feet of water per second is currently flowing down the river due to snowmelt, compared with 18,000 CFS during the same period last year.

"Last year was epic," McDonnell said. "It was quite fun and exhilarating, but a lot of people shy away from the higher flows."