What causes the famous Qiantang tidal bore?

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Zhejiang Dashan (“Great Mercy”) Pagoda, Shaoxing, Zhejiang province, China.

The famous Qiantang tidal bore in Zhejiang, China, is caused by the interaction between a large tidal range and the geography of the river's mouth.1

A tidal bore forms when the incoming tide is confined to a narrow channel, causing the water to rush upstream as a distinct "wall of water" or a series of breaking waves. This phenomenon occurs specifically during spring tides—when the gravitational pull of the Sun and Moon reinforce each other—and does not occur during neap tides.1

In the case of the Qiantang River, the funnel-like shape of the estuary concentrates the incoming tidal energy, resulting in a powerful bore that can reach heights of nearly 9 meters (about 29 feet). As the bore passes, it abruptly reverses the river's flow, pushing water upstream with significant momentum.1