The Gironde Estuary, formed by the confluence of the Garonne and Dordogne rivers, is the largest estuary in Europe at almost 50 miles (80 km) long and up to 7 miles (11 km) wide. The estuary’s average discharge rate into the Atlantic is 265,000 gallons (1 million liters) per second. It has a large tidal range, of up to 16 ft (5 m) during periods of spring tide, and the strong tidal currents in the estuary, as well as numerous sand banks, tend to hamper navigation. One of the Gironde’s most impressive features is its tidal bore—a large, wall-like wave at the leading edge of the incoming tide—known locally as the Mascaret. Occurring with each flood tide at the time of spring tides (that is, twice daily for a few days every two weeks), the bore surges from the Gironde upstream into its narrower tributaries. On the Garonne, the Mascaret sometimes forms a barreling wave, which can reach a height of 5 ft (1.5 m) and tends to break and reform.
The Gironde is an important artery of the Bordeaux wine region and a rich source of eels and a wide variety of shellfish, which feature on local restaurant menus. Wild sturgeon (the source of caviar) were once also plentiful in the estuary, and although their numbers have declined due to overfishing, they are still farmed in small numbers.