World's Largest Tidal Bore, Qiantang River, China

Firstime, in September 2008 a group American surfers convinced the Chinese government to allow them to surf a section the river. Personal watercraft were used to safely transport the surfers in and out the tidal bore.
Tides are the rise and fall sea levels caused by the combined effects the gravitational forces exerted by the Moon and the Sun and the rotation the Earth. Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude the tides at the coast are influenced by the alignment the Sun and Moon, by the pattern tides in the deep ocean and by the shape the coastline and near-shore bathymetry.