Tidal bore

A tidal bore is a sudden rise in the water level of an estuary just before or just after high tide, under the effect of the incoming tide.

During spring tides when the incoming tide meets the descending river current, a powerful wave is formed which travels upstream at high speed.

This phenomenon only exists in certain types of estuary (around twenty in the world) with large mouths and a sudden funnel-like reduction in width and depth, with a bed punctuated by sand banks. The Severn bore in the UK is an example of a tidal bore.