Tidal bore

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The tidal bore in Upper Cook Inlet, Alaska

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 (typically more than 20 feet between high and low water), 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:

• 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, near Sri Aman, Malaysia. The tidal bore is locally known as benak.

• 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.

Tidal bore on the Petitcodiac River

• 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.

Most rivers off the upper Bay of Fundy between Nova Scotia and New Brunswick have tidal bores. These include the River Hebert and Maccan Rivers, the St. Croix River in the Minas Basin, and the Salmon River in Truro.

• 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 Welland, England
• River Kent, England
• River Great Ouse, England
• River Eden, England
• River Esk, Scotland
• River Nith, 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.

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

See also

• Hydraulic jump

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