The Ohio River is a left (east) and largest tributary by volume of the Mississippi River in the United States. At the confluence, the Ohio is considerably bigger than the Mississippi, measured by long-term mean discharge. The Ohio River at Cairo is 281,500 cu ft/s (7,960 m3/s); and the Mississippi River at Thebes, Illinois, which is upstream of the confluence, is 208,200 cu ft/s (5,897 m3/s). The Ohio River flow is greater than that of the Mississippi River so hydrologically, the Ohio River is the main stream of the river system.