YOU SHOULDN'T DRINK the water in China. My biggest chance comes on day three of the rafting trip, near the northernmost tip of the Great Bend, where a landslide pinches the Yangtze into a frothing chute called Judgment Day. The river accelerates, hits a jagged wall, spits right, and then tumbles left down a boulder-strewn exit.

In West Virginia, this "feature" would be labeled Class IV, but Willie pours scorn on the rapid. "Class III, Class III," he chants as we stand atop a slick boulder in an ice-cold rain, scouting. The crew is miserable after hours of gritty paddling into headwinds and spitting rain, our toes marinating in chilled Yangtze. From atop the boulder, Willie calls for a vote: Do we want the safe ride, down the river's right side? Or the sexy trip straight down the frothing middle?

Davo and Des, the California river rats, blurt it out: "Middle! Middle!"

I say nothing. Elissa says nothing. Owen and Roger, waiting back in the raft, can't hear him over the rapids and don't say anything. It's a perfect Chinese democracy: one fearless leader, two yes-men, and four obedient masses.

Willie spurs us into the current, and we slide sideways down a 50-foot tongue of river, smooth as jade, and then pivot to face a freight train of standing waves. We are sucked down, buried, and thrown out of the foam three times, and all come down in the boat but one.

Owen is overboard, partly; his head and torso are underwater, his feet kicking in the air. Des grabs a leg. Four of us paddle hard for the right. We bounce off the wall anyway, sweep right, and paddle hard to the left, Owen admiring fish the whole way. Finally we pull him in. But as Owen comes in, Des goes over. Again the legs wave, and Des rights himself, grinning. We drop into an eddy to await the next victims, the boat full of laughter.

You'd think the river was suddenly ten degrees warmer and the sun had come out.

We spend that night, like the rest, on the powdery banks of the Yangtze. I've taken to throwing down my sleeping bag wherever Mr. Exercise pitches his little blue assault tent. We can't communicate, except in the ways that matter, and I help him wrestle with the buttons on his GPS unit, which he keeps sealed in a condom, the Chinese drybag. (The signal is usually blocked, but we can't tell if it is the canyon or the rubber.) Mr. Exercise is the only SWaFU with wilderness cred. Despite his jokes—he teaches the Americans to shout "Wo cao!" during big drops without explaining that it means "Fuck me!"—he is making a careful study of the rafting business. Through an interpreter, I ask if he is ready to become a guide. He answers with a humble "Mei you." No way.

"We formed a concept of real rafting this time," he explains. "But we need to experience more. It is the same as mountain climbing. You have to learn the techniques, like reading the water and the weather." The voluble Chen Hai, by contrast, nips into the whiskey and announces that he is ready for Class V. As soon as he finds his glasses. Which are on his head.

Two dozen children clamber down from a nearby village, called Three Rivers. The guides call it Three Thieves, since they were robbed during a previous visit. This time the well-behaved children are under the supervision of four mothers in slacks and sandals. They call themselves the Lulu. The SWaFUs, all Han Chinese, are thunderstruck to discover that none of the Lulu speak Mandarin, the national language. Dragon shakes his head. Until today, he thought that "90 percent, at least," of Chinese spoke Mandarin.

Isolated cultures like the Lulu are being fragmented by this rising tide of asphalt and infrastructure. Good roads now rise up to towns at 11,000 feet that were utterly inaccessible during my 1988 Yunnan tour. Back then, a Tibetan settlement called Gyalthang had already been renamed Zhongdian by the Chinese. In 2001, it was rechristened, again, after market research, as Shangri-La. Tourism rose from 20,000 annual visitors in 2000 to 2.6 million in 2005, but any place that calls itself "the real Shangri-La" doesn't know what it is.

Within the Great Bend itself, in the gorgeous town of Lijiang, the tenth-century tiled houses of the Naxi, Yunnan's largest minority, have been converted into souvenir shops and karaoke bars. The Naxi live in high-rises on the outskirts of town. Even Kunming has grown to 4.5 million people, with a science-fiction skyline and malls dedicated to Hermès, Gucci, and Lacoste. Before the trip, I spent days searching out the slabs of warm onion bread that I remembered eating in the Muslim quarter, but the onion bread was gone, along with the entire Muslim quarter. After 18 years, I arrived on the very afternoon that a bulldozer flattened the last row of Arabesque houses.

Still, there are the Lulu, tucked into their own fold of the river. After dinner that night, there is a singing contest around a driftwood bonfire. The children are splendid, of course. Mr. Li, a skinny Yangtze boatman whom Jim and Jed hired after watching him row past on a previous trip, belts out a tragic lament about rivermen and forgotten lovers. And Owen, who works as an actor back in San Francisco, silences everyone with a haunting version of "Raglan Road" that leaves even stoic Mr. Li staring into the fire.

The Lulu children climb back to their village. In the darkness, I hear the guides moving about, carefully checking that all our equipment is still here.