In a few places on Earth, local geography and tidal conditions align to cause a phenomenon called a tidal bore: An incoming high tide collides with the outgoing tide in a narrow channel, generating a turbulent wave front. The shape and size of a bore tide varies greatly, from gentle undulations to a powerful single wave rolling over any obstacle in its path. China's Qiantang River claims the highest bore, up to 9 meters (30 ft) high. In other places, such as Alaska's Turnagain Arm, Brazil's Amazon River, and the River Severn in the UK, surfers paddle out and try to catch the bore tides for a quick trip upstream.
A crowd of Chinese tourists run away as a tidal bore breaks over a wall along the Qiangtang River in Haining, Zhejiang province, China, on on August 31, 2011. Visitors gathered to experience the Qianjiang Tidal Bore, an annual tradition for the residents living nearby.#
A remote controlled helicopter hovers over the Qiantang River as tourists gather on the river bank to see the soaring tide in Haining, Zhejiang province, on September 13, 2011.#
Spectators flee as waves created by a tidal bore crash over a barrier on the Qiantang river at Hangzhou, in east China's Zhejiang province, on August 31, 2011.#
Policemen and residents run as waves from a tidal bore surge past a barrier on the banks of Qiantang River in Haining, China, on August 31, 2011. As Typhoon Nanmadol approached eastern China, the tides and waves in Qiantang River recorded their highest level in 10 years, local media reported.#
A crowd of Chinese tourists run away as a tidal bore breaks through the dam by the Qiangtang River in Haining, east China's Zhejiang province on August 31, 2011. About 20 people were injured when they were caught too close to the river while viewing the annual tidal bore, which occurs when sea water from an unusually high tide funnels into the river, creating high waves.#
South of Anchorage, Alaska, the biggest bore tide of the summer roared into Turnagain Arm on June 5, 2012. Here, a kayaker awaits the bore tide to ride it.#
Brazilian surfer Savio Carneiro celebrates as catches the thunderous "Pororoca" tidal bore wave during the national pororoca circuit final on the Mearim River, some 30 km inland, in the Amazon jungle near the northern Brazilian city of Arari, on April 19, 2003. The term "Pororoca" comes from the Amazonian indigenous term meaning "destroyer, great blast." The feared and thunderous waves have capsized boats and washed away most anything in its path.#
Brazilian surfer Rogerio Dantas jumps into the Mearim River to catch the thunderous Pororoca tidal bore wave, near the northern Brazilian city of Arari, on April 17, 2003.#
A view of the Severn Bore, sweeping down the river Severn between Stonebench and Minsterworth, in Gloucestershire, England, on September 3, 1936. At times the waves reached a height of 20 feet.#
Surfers ride the Severn Bore near Newnham along the River Severn on March 2, 2010 in Gloucestershire, England. Surfers from around the world flocked to ride the Severn bore, a natural tidal phenomenon that pushes a 4ft wave up the Severn Estuary in Gloucestershire.#
A tidal wave hits a bank along the Qiantang River on August 22, 2013 in Haining, China. Typhoon Trami landed in Fujian province earlier and led to higher-than-normal tides, gales and heavy rainfalls in east China.#
Visitors run away from a tidal bore wave as it surges over a barrier on the banks of Qiantang River, in Hangzhou Zhejiang province, on August 25, 2013.#
People struggle as waves from a tidal bore pour over a barrier on the banks of Qiantang River in Haining, China, on August 22, 2013. As Typhoon Trami landed in eastern China, tidal level in the Qiantang River was recorded at 6.6 meters high with a surge reaching 1.3 meter high, local media reported.#
Spectators get to their feet, after being swept down a hill by huge waves while watching tides of Qiantang River on August 22, 2013 in Haining, China.#
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