The Mobile River is located in southern Alabama in the United States. Formed out of the confluence of the Tombigbee and Alabama rivers, the approximately 45-mile-long (72 km) river drains an area of 44,000 square miles (110,000 km2) of Alabama, with a watershed extending into Mississippi, Georgia, and Tennessee. Its drainage basin is the fourth-largest of primary stream drainage basins entirely in the United States. The river has historically provided the principal navigational access for Alabama. Since construction of the Tennessee-Tombigbee Waterway, it also provides an alternative route into the Ohio River watershed.