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.