The question seemed straight-forward but the result was unexpected.

“Do you want to surf?" Adam Crom had asked from his perch on the back of our blue five-person raft. Four voices sang out “Yes!" in anticipation.

“Right turn, right turn, right… now stop!" came the staccato commands from our confident Sierra Mac boatman. As we pulled our dripping paddles free of the water, the back of the raft slid down a tall pillow of white foam, driving the boat's bluff nose inexorably into the tongue of smooth green Tuolumne flowing into the surf hole. The irresistible river rolled fast and chilly into the front of the boat, filling it until my perch on the bow was a good foot below the surface.

I marveled at the odd river's-eye view as Crom held the boat steady for a stretched-out thirty seconds. Then he dipped an oar, peeling us out with a thrilling lurch that spun the boat's left side in the air. We landed with a thump, facing downstream.

We were floating the main Tuolumne run, a class IV thrill ride that tumbles 18 miles from Meral's Pool to the tall bridge at Ward's Ferry. In that stretch the river sheds 780 feet, charging through nearly 20 fast-action rapids with names such as Rock Garden, Nemesis, and Sunderland's Chute.

From high above, they hadn't looked the least fearsome. From a narrow seat in Sierra Mac's creaky old school bus, the river at the bottom of the narrow pine shrouded canyon was a silver thread frosted with a near-continual line of white foam – one rapid after another in quick succession.

The big daddy of them all is Clavey Falls, where the Tuolumne plunges down a pair of dizzying 8-foot steps. It was the only rapid supremely confident boatman Crom scouted.

“If you fall out, swim toward the left side," he said matter of factly, pointing at a whipped torrent scraping a wall of granite. It looked like a Jacuzzi jet only a million times larger, colder, and not the least inviting.

We piled back in the raft and floated faster and faster toward our fate. As we reached the crux of the fall, “Down!" commanded Crom, and four bodies huddled in the bottom of the raft. A couple moments of free fall and one tremendous soul-drenching splash later, we eddied out breathless with adrenalized laughter.

I licked the spray from my lips and tasted the grandeur of Yosemite National Park – a drink of Sierra Champagne in the words of Aaron Cavagnolo, another Sierra Mac boatman. This fine sparkling water originates high in Yosemite's granite fastness. Briefly held back by O’Shaughnessy Dam, it bursts forth from drowned Hetch Hetchy Valley, boisterously celebrating its escape down the wicked class V Cherry Creek run and on into the classic main Tuolumne fun run.

The Main Tuolumne Run is typically boatable from March through September due to scheduled releases from Hetch Hetchy reservoir. Trips with Sierra Mac ( start at $225 for one day. Camping along the way ($430 for 2 days; $540 for 3) allows ample time to relax, explore the area's gold rush history, and swim in warm side- canyon streams.

The main Tuolumne nearly suffered Hetch Hetchy's fate.

“We fought off three other dam projects," said Martin McDonnell, Sierra Mac's owner and a founding board member of the Tuolumne River Trust. In part due to the Trust's efforts, the Tuolumne was designated a National Wild and Scenic River in 1984. McDonnell and his company have run raft trips down the California's premier whitewater jewel since 1969.

Back on the river 18 miles downstream of the put-in, the Tuolumne gently relinquished our raft to the still waters of Don Pedro reservoir. As Crom slowly rowed the final stretch to the take-out at Ward's Ferry, the song of the free-flowing river faded, leaving the crisp taste of Sierra champagne to linger on the tongue.