What is a tidal bore?

A tidal bore forms in certain rivers and estuaries where the incoming tide is funnelled into a narrowing, shallowing channel. As the rising tide is squeezed, the leading edge steepens into a single wave, or a series of waves, that rushes upstream against the river normal downstream flow. Tidal bores only occur in places with a large tidal range and the right shape of estuary, so they are relatively rare worldwide. Some bores are small ripples, while others form powerful waves surfers and boaters can ride for kilometres. Famous examples include the Qiantang River bore in China and the Severn bore in England.