The rising tide coming in from Hangzhou Bay is funneled by the shape of the Qiantang River into the world's largest tidal bore, a long breaking wave known as the Silver Dragon which sometimes reaches 30 feet high.
The tidal bore happens during the spring tide with every full moon, but is strongest in the fall, when the Tide-Watching Festival is held on the 18th day of the 8th month in the Chinese calendar. The festival can attract up to 170,000 tide-watchers and has been celebrated for hundreds of years. Indeed, observations of the regular occurrence of the Silver Dragon produced the oldest known tide table in 1056 AD to help tourists arrive in time to see the fast-moving wave.
The bore is first seen as a distant stroke of silver on the horizon. Then the rushing sound of a great waterfall can be heard growing steadily louder as it approaches at a speed up to 25 miles per hour. Some years a nearby typhoon has caused the waves to crash against the river banks violently, sucking tourists off the sea walls in its wake. After the Silver Dragon churns past, the placid river current is now replaced by choppy and turbulent ocean waves, suddenly many feet higher and completely transformed in character.