<P> The Mississippi River is known as the Middle Mississippi from the Upper Mississippi River's confluence with the Missouri River at St. Louis, Missouri, for 190 miles (310 km) to its confluence with the Ohio River at Cairo, Illinois . </P> <P> The Middle Mississippi is relatively free - flowing . From St. Louis to the Ohio River confluence, the Middle Mississippi falls 220 feet (67 m) over 180 miles (290 km) for an average rate of 1.2 feet per mile (23 cm / km). At its confluence with the Ohio River, the Middle Mississippi is 315 feet (96 m) above sea level . Apart from the Missouri and Meramec rivers of Missouri and the Kaskaskia River of Illinois, no major tributaries enter the Middle Mississippi River . </P> <P> The Mississippi River is called the Lower Mississippi River from its confluence with the Ohio River to its mouth at the Gulf of Mexico, a distance of about 1,000 miles (1,600 km). At the confluence of the Ohio and the Middle Mississippi, the long - term mean discharge of the Ohio at Cairo, Illinois is 281,500 cubic feet per second (7,970 cubic metres per second), while the long - term mean discharge of the Mississippi at Thebes, Illinois (just upriver from Cairo) is 208,200 cu ft / s (5,900 m / s). Thus, by volume, the main branch of the Mississippi River system at Cairo can be considered to be the Ohio River (and the Allegheny River further upstream), rather than the Middle Mississippi . </P> <P> In addition to the Ohio River, the major tributaries of the Lower Mississippi River are the White River, flowing in at the White River National Wildlife Refuge in east central Arkansas; the Arkansas River, joining the Mississippi at Arkansas Post; the Big Black River in Mississippi; and the Yazoo River, meeting the Mississippi at Vicksburg, Mississippi . The widest point of the Mississippi River is in the Lower Mississippi portion where it exceeds 1 mile (1.6 km) in width in several places . </P>

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