Tidal Bore

A tidal bore is a wave of translation, a moving step in water level, that travels upstream against the current as a rising tide enters a funnel-shaped estuary or river. It forms where a large tidal range, typically above 6 meters, meets a shallow, narrowing, gently sloped channel, so the leading edge of the flood steepens into a single surge or undular train. The Qiantang in China, the Severn in England, and the Amazon Pororoca are classic examples. The bore can run several knots and a meter or more high, a hazard to small craft and a fixed timing for upstream navigation.