The "Silver Dragon." Located on the Qiantang river in Hangzhou China, the world's largest tidal bore creates a wave that runs up river allowing surfers to ride it for miles.
Hubert Chanson, a professor of hydraulic engineering at the University of Queensland in Australia, has documented 190 tidal bores around the world. He says the two biggest are the Qiantang and the Pororoca on the Amazon.
Chanson says bores form on rivers with big tidal swings from, say, 13 to nearly 20 feet. When the gravitational pull of a full or new moon raises the water level at the river's mouth especially high, it triggers a wave that rolls upstream.
One reason the Qiantang bore is so big is that the triangular-shaped Hangzhou Bay acts like a funnel, amplifying the wave as it moves inland. The bore takes various shapes. Sometimes, it has a sloping face a surfer can cut across. At others, it can break into a surging, foamy, nearly two-story wall of water.
The bore has so much power, it can crash over sea walls, wiping out crowds. In 1993, nearly 60 people were killed while watching. Chanson says the first person on record to surf a tidal bore was a British soldier who rode the U.K.'s Severn River in the 1950s.
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