A tidal bore (or just bore, or eagre) 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 or narrow bay against the direction of the current. As such, it is a true tidal wave (not to be confused with a tsunami).

Bores occur in relatively few locations worldwide, usually in areas with a large tidal range, and where incoming tides are funnelled into a shallow, narrowing river via a broad bay. The funnel-like shape not only increases the height of the tide, but it can also decrease the duration of the flood tide down to a point where the flood appears as a sudden increase in the water level.

Bores take on various forms, ranging from a single breaking wavefront—effectively a shock wave—to ‘undular bores’ comprising a smooth wavefront followed by a train of solitary waves (solitons). 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, India, 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).
• Batang Lupar or Lupar River, Malaysia. The tidal bore is locally known as benak.

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, formerly the highest bore in North America, up to 7.5 metres (25 feet) high. Reduced to little more than a ripple due to causeway construction and excessive siltation.
• 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.
• Saltstraumen near Bodø, Norway, claimed to be the strongest tidal current in the world.

Australia

• Styx River, Queensland, Australia
• Daly River, Northern Territory, Australia Gezeitenwelle

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