river in southern Alabama, U.S. It is formed by the Coosa (Coosa River) and Tallapoosa (Tallapoosa River) rivers, 7 miles (11 km) northeast of Montgomery, winds westward to Selma, and then flows southward. Its navigable length is 305 miles (491 km), and the river drains 22,800 square miles (59,050 square km). It receives its chief tributary, the Cahaba, about 10 miles (16 km) southwest of Selma. The Alabama is joined 45 miles (72 km) north of Mobile by the Tombigbee (Tombigbee River) to form the Mobile (Mobile River) and Tensaw rivers, which flow into Mobile Bay, an arm of the Gulf of Mexico. Mobile and Montgomery became major cities largely because they were on this important traffic artery. The Coosa-Alabama river system, with various locks and dams, has been a major factor in the economic development of the state.