In a few funnel-shaped rivers the incoming tide arrives as a single breaking wave travelling upstream — up to nine metres high on the Qiantang in China, where people have watched it for two thousand years.

It happens when a large tidal range is forced into a narrowing, shallowing channel: the front of the flood catches up with itself and steepens. Bores run the Severn, the Amazon and the Seine before it was dredged, and they are strongest at spring tides, so the calendar of a bore is the calendar of the Moon.