A tidal bore is a body of water that travels upstream at twice or three times the speed of a normal tidal current. Tidal bores are caused by a combination of lunar and solar gravity and a sandy river bottom. As the massive wave advances up the river, the tide is produced by fresh water that has passed down further and been collected and returned ahead of the incoming tide. The velocity of the advancing saltwater is thus used to calculate how much water comprises the regular tidal flow and how much is caused by the bore effect.

In a layperson's terms, tidal waters are forced in through the bell-shaped mouth of the river, unable to flow out, which creates a massive, bulbous wave of water.

The Amazon River in Brazil has the most famous tidal bore, which stretches 14km across and 3.5 meters high. Similarly impressive are the regular tidal bores at the Bay of Fundy in Nova Scotia, Canada, which can reach approximately 15 meters.

Zhejiang Province's bore reaches between 3.5-8 meters, with the 18th day of the eighth lunar month as the best time to catch the wave. As the tide snakes its way along the Qiantang River, it encounters such obstacles as mountains, dams, and ox-bow bends, forming columns of water between 2 and 10 meters height.

  Tide-watching tradition

The tradition of watching the tidal bore on the Qiantang River dates back more than 2,200 years. The custom first appeared during the first century BC and became popular in the Tang Dynasty (618-907).

During the Southern Song Dynasty (420-479), the imperial court would arrange for a marine inspection ceremony, with naval vessels racing ahead of the advancing wall of water. More recently, Commander W Usborne Moore of the British Navy (1888-1892) reported waves up to 3.5 meters high.

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