Garonne River, a river of southwestern France. It is 402 miles (647 km) long. From its source in the Pyrenees Mountains in Spain, it flows northeastward to Toulouse, then northwestward; near Bordeaux it joins the Dordogne River to form an estuary called the Gironde. The estuary empties into the Bay of Biscay. Major tributaries of the Garonne include the Gers, Baise, Tarn, and Lot rivers. The Garonne and its tributaries drain the Aquitaine Basin of southwestern France.