Garonne tidal bore research. Researcher Mario Lepage with fish fry collected before the peak of the Garonne tidal bore. A tidal bore occurs when a large tidal range causes water to surge into a funnel-shaped bay, river, or estuary. This is the Gironde estuary in south-western France, at the meeting of the Dordogne and Garonne rivers. This research is being carried out by Lepage for CEMAGRAF, the French public agricultural and environmental research institute. The study is examining the effect of the tidal bore on fish migration and breeding patterns. Photographed near Podensac, France, on 6 September 2010.