Director Yang Chao says his film Chang Jiang Tu (Crosscurrent), shown in competition on Monday at the Berlin International Film Festival, is like a love poem for one of the most important rivers in China - and also one of its most damaged.

The film blends elements of the real and the surreal as it follows a quest by the young captain Gao Chun, played by Qin Hao, as he steers his decrepit hulk of a freighter up the 6,300-kilometer river to deliver a mysterious cargo.

He is also in pursuit of a beautiful young woman, An Lu (Xin Zhilei), who may or may not be a phantasm, and who appears at various places along the river, sometimes to make love to him, at other times to vanish from sight.

During the voyage, Gao reads from a book of poetry that is hidden away in a special compartment on the boat, while the screen flashes verses from famous poets of Chinese history.

"There's a big classical tradition of Chinese poetry and successive Chinese poets from the Tang Dynasty (AD 618-907) through the other dynasties to the present day have used a variety of approaches to describe, to talk about the Yangtze River," Yang says.

"But for people in China the Yangtze River doesn't just exist on that level of culture and poetry. In the different eras of Chinese history, it also has been the most prosperous belt of China. So in a way it's a bridge between tradition and present day."

In the modern age, the river has been drastically changed by the development of massive cities along its banks, and by the construction of the Three Gorges Dam, the world's largest power station in terms of installed capacity.

The film shows the rusty hulk, named Guang 039, going through the huge, ultramodern locks at Three Gorges in order to continue its trip upriver. At the same time, the film shows a map of the towns and places that were submerged and vanished when the dam was completed.

"Perhaps the single most important metaphor is the rising water level which was a consequence of the Three Gorges Dam project and does not only submerge a lot of ancient sites but also it serves to block the love story in the film from going any further," Yang says.

"That's a key metaphor that we see again and again in Chinese culture: The beautiful being changed by the passage of time - the changes that time brings about not only to cultural things but also to love."