Tidal bore
Turnagain-bore.jpg
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. 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, 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
Tidal_bore.jpg
- 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.
Oceania
- Styx River, Queensland, Australia
- Daly River, Northern Territory, Australia