A tidal bore (or just bore) is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travel up a river against the direction of the current. As such, it is a true tidal wave (not to be confused with a tsunami).
Bores occur in a relatively few locations worldwide, in areas with a large tidal range, only on certain tides, and when an incoming tide is funnelled into a shallow, narrowing river via a broad bay. A bore is a natural example of a self-reinforcing solitary wave or soliton. Larger bores can be particularly dangerous for shipping, but also present a challenge to surfers.
The word bore derives through Old English from the Old Norse word bara, meaning a wave or swell.
Rivers that have been known to exhibit bores include:
- Amazon River, up to 4m (12 feet) high, running at up to 25 km per hour (15 miles per hour). It is known locally as the pororoca.
