The Ohio River, which streams westward from Pittsburgh, Pennsylvania, to Cairo, Illinois, is the largest tributary, by volume, of the Mississippi River in the United States. At the confluence, the Ohio is considerably bigger than the Mississippi (Ohio at Cairo: 281,500 cu ft/s (7,960 m³/s); Mississippi at Thebes: 208,200 cu ft/s (5,897 m³/s)) and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream.