The Ohio River is a major tributary of the Kentucky River, which is located in the southeastern United States. The Ohio River is 981 miles long and is the largest tributary of the Mississippi River. It is formed by the confluence of the Allegheny and Monongahela rivers in Pittsburgh, Pennsylvania. The Ohio River flows through six states before joining the Mississippi River near Cairo, Illinois. The Kentucky River is a tributary of the Ohio River, originating in the Appalachian Mountains of eastern Kentucky. It is approximately 260 miles long and flows through the cities of Lexington, Frankfort, and Louisville before joining the Ohio River.