river formed by the confluence of the Tombigbee (Tombigbee River) and Alabama (Alabama River) rivers, southwestern Alabama, U.S. It enters Mobile Bay after a southerly course of 45 miles (72 km) through the delta region. With its tributaries it drains some 44,000 square miles (114,000 square km), making it the sixth largest river basin in the United States. Mobile, on the west side of the river at the head of the bay, is its chief city and a centre for commercial river traffic.