The Ohio River is a major tributary of the Mississippi River, and it forms part of the border between Ohio and Kentucky. The Ohio River is 981 miles long, making it the longest river in the United States that flows entirely within one state. It is also the second-largest tributary of the Mississippi River by volume, with an average discharge of over 500, 000 cubic feet per second. The Ohio River is an important source of water for both states, providing drinking water, irrigation, and transportation. The Ohio River is also home to many species of fish, birds, and other wildlife.