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:

Asia

• Brahmaputra, Bangladesh
• Indus River, Pakistan
• Qiantang River, China, which has the world's largest bore, up to 9 metres (30 feet) high, travelling at up to 40 km per hour (25 miles an hour).

South America

• 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.

North America

• Petitcodiac River in the Bay of Fundy, New Brunswick, Canada, the highest bore in North America, up to 7.5 metres (25 feet) high.
• Shubenacadie River , also off the Bay of Fundy, Nova Scotia.
• Turnagain arm of Cook Inlet, Alaska. Up to 2 metres (6 feet) and 20 km per hour.

Europe

• River Dee, Wales / England
• River Mersey, England
• River Severn, Wales / England up to 2 metres (7 feet) high
• River Trent, England and other tributaries of the Humber Estuary
• River Parrett, England
• River Kent, England
• River Great Ouse, England
• River Eden, England
• River Esk, Scotland

• Dordogne River, France
• Garonne River, France
• Seine, France, had a significant bore ("le mascaret") until the 1960s, since then it has been practically eliminated by dredging.