Canoes on the Garonne tidal bore, aerial photograph. Caused by the twice-monthly peak tides (spring tides), this phenomenon is where a large tidal range occurs in a funnel-shaped bay, river, or estuary. In this case, the tidal bore occurs in the Gironde estuary in south-western France, at the meeting of the Dordogne and Garonne rivers. Locally called the mascaret, the waves produced depend on the river topography. Surfers and other sports enthusiasts ride on the waves, which can be 1.5 metres in height, travelling at a speed of 12 kilometres an hour. Photographed on 10 September 2010.