Largest tidal bore

Tidal bores are formed when the front of the incoming tide forms a wave or series of waves that travels up a river against its direction of flow. This phenomenon occurs in several dozen locations around the world but by far the largest and most spectacular happens in the Qiantang River, China, where it can form a massive wave of water up to 9 m (30 ft) high and travelling upstream at up to 40 km/hr (25 mph).

The largest of these events occur in the autumn around the occurrence of "spring tides" associated with the best alignment of Sun and Moon to produce large tides