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 m
3/s); Mississippi at
Thebes: 208,200 cu ft/s (5,897 m
3/s)) and, thus, is hydrologically the main stream of the whole river system, including the
Allegheny River further upstream.