The Ohio River is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi (Ohio at Cairo: 281,500 cfs (7,960 m3/s); Mississippi at Thebes: 208,200 cfs (5,897 m3/s)) and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream. It is approximately 981 miles (1,580 km) long and is located in the Eastern United States.