estuary on the Bay of Biscay, in Gironde département, Aquitaine région, southwestern France, formed by the confluence of the Garonne (Garonne River) and Dordogne (Dordogne River) rivers (qq.v.). It trends from southeast to northwest for about 45 miles (72 km) and is navigable for oceangoing vessels, although it has sandbanks and strong tides.

Various specialist facilities of the port of Bordeaux are located along the estuary. Ambès, at the confluence of the Dordogne and the Garonne, handles oil products and other bulk liquids; Blaye, on the right bank, specializes in the export of cereals; Pauillac is an oil port; and Le Verdun-sur-Mer is a container terminal.