The Ohio River is formed by the confluence of the Allegheny and Monongahela Rivers in Pittsburgh. From there, it flows northwest before making an abrupt turn to the southwest at the Ohio, West Virginia and Pennsylvania border. The Ohio then follows a roughly west-northwest course until Cincinnati, before bending southwest for the remainder of its journey through the US Midwest and joining the Mississippi at Cairo, Illinois. The 981 mile – 1,579 km – river flows through or along the border of six states; its basin includes parts of 15 states. The Ohio’s largest tributary is the Tennessee River.