When an enormous lock creaks to life in the opening scene of “Up the Yangtze,” its hulking slabs of metal shifting like tectonic plates, it’s almost as if the rapid transformation of China’s powerhouse river is unfolding in geologic time. It’s an appropriate overture, as Yung Chang’s lovely, unhurried documentary goes on to extract some timeless truths from China’s latest great leap forward.

After swimming the Yangtze’s great width, Mao Zedong was stirred in 1956 to compose a poem about damming it up. “Great plans are afoot,” he wrote, and they still are today. In fact, the planned Three Gorges Dam, which will soon be the world’s largest hydroelectric station, makes Mao’s dream of “turning a deep chasm into a thoroughfare” seem tentative by comparison.

Mr. Chang, a Canadian of Chinese origin, first observed the changes being wrought by the dam project while on vacation with his family in 2002. Inspired, as he explains in voice-over, by the idea of “a brand new country being created,” he spent much of 2006 filming two Chinese youths working on a Yangtze cruise boat catering to Western tourists. As Chen Bo Yu (“Jerry,” to passengers) and Yu Shui (“Cindy”) struggle with English phrases and learn how to spend pocket money, “Up the Yangtze,” which opens Friday at IFC Center, coaxes a resonant metaphor out of this vessel of foreign values carving a path into the future.

It also dramatizes the upheaval such progress leaves in its wake. Rising waters caused by the dam project will have displaced some 2 million people by the time the project is completed in 2011. Particularly moving is the quandary of Yu Shui’s parents, who make their daughter (who prefers high school) take a job on the boat because the dam will soon force her illiterate father to abandon his crops. They live modestly as it is, in a tumbledown shack, and their shame poignantly deepens the generational divide.

Miserable at first at her new job, Yu Shui gradually learns the ropes. Mr. Chang’s camera seizes on her conflicting emotions: She’s homesick, yet mortified when her boss invites her ill-dressed parents to the boat’s lounge and tells them to have faith in the new China. They nod politely, confused by the English phrases he sprinkles into his Mandarin. From behind, we see them retreat down the gangplank; it’s a shot steeped in sadness.

Mr. Chang betrays a wry sense of humor, though, in a scene in which the boat’s manager lectures his staff. The Westerners once referred to as “foreign devils” are now jolly good for business, he explains — assuming one knows how to put them at ease. Refer to overweight passengers as “plump,” he instructs, and “avoid the issue of Northern Ireland.”

Brilliantly, Mr. Chang waits until a kitschy dress-up night before he asks the foreign passengers about their impressions of China. Many know the scenery will soon disappear, and have come for that reason. Still, they seem oblivious to what’s happening onshore. A guide leading a tour of new public housing cheerfully avoids discussions of unequal treatment of relocatees. Challenged by an American woman, he responds: “All of them are happy!”

But a shopkeeper confides that those who can’t afford to bribe officials get nothing. Then, startlingly, he breaks down. “China’s too hard for common people,” he sobs.

None of that concerns Chen Bo Yu too much. Ambitious and handsome, he’s one of millions of middle-class young men in China (many of whom are only children) who see the world as their oyster. “I don’t want a flat and tasteless life,” he declares. He seems unstoppable. But his selfish attitude eventually brings grim consequences — a reminder of the uneasy mix of cultural factors at play as China surges ahead.

“Imagine the Grand Canyon being turned into a Great Lake,” Mr. Chang says in voice-over of the dam project. But rather than simply registering dismay, “Up the Yangtze” quietly takes in many angles, both emotional and visual. It’s hard not to respond to the director’s images of Yu Shui’s house being swallowed up by the river. Time-lapse photos dissolve into one another as the water rises: As the ground beneath her home begins to soften, the shack begins to collapse, then falls flat, then disappears altogether and becomes part of history. Like much of China’s natural landscape, the Yangtze was harnessed long ago. A more relevant question, which this film almost whispers, is whether its tamers will ever be tamed.

At IFC Center (323 Sixth Ave. at West 3rd Street, 212-924-7771).