Lower Mississippi River

The Mississippi River is called the Lower Mississippi River from its confluence with the Ohio River to its mouth at the Gulf of Mexico.

Measured by water volume, the Lower Mississippi's primary branch is the Ohio River. At the confluence of the Ohio and the Middle Mississippi, the Ohio is the bigger river, with its long-term mean discharge at Cairo, Illinois being 281,500 cu ft/s (7,970 m³/s), while the long-term mean discharge of the Mississippi at Thebes, Illinois (just upriver from Cairo) is 208,200 cu ft/s (5,900 m³/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.

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; the Yazoo River, meeting the Mississippi at Vicksburg, Mississippi; and the Red River in Louisiana.

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.

Due to deliberate water diversion at the Old River Control Structure in Louisiana, the Atchafalaya River in Louisiana is now a major distributary of the Mississippi River, with 30% of the Mississippi's flow routinely entering the Gulf of Mexico by this route, rather than continuing down the Mississippi's current channel past Baton Rouge and New Orleans on a longer route to the Gulf.

Middle Mississippi River

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.

The Middle Mississippi is a relatively free-flowing river. From St. Louis to the Ohio River confluence, the Middle Mississippi falls a total of 220 feet (67 m) over a distance of 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 River, no major tributaries enter the Middle Mississippi River.

Measured by length, the Middle Mississippi's primary branch is the Missouri River, not the Upper Mississippi, whether or not additional tributaries upstream are considered. Thus, by length, the main branch of the Mississippi River system at St. Louis can be considered to be the Missouri River, rather than the Upper Mississippi. By taking the longer branch at each significant fork, this continuous but multiply named waterway can be identified and measured. One name for it is the Lower & Middle Mississippi-Missouri-Jefferson-Beaverhead-Red Rock-Hellroaring Creek River. The name "Great American River" has also been suggested for this longest American waterway.

Upper Mississippi River

The Mississippi River is known as the Upper Mississippi from its headwaters to its confluence with the Missouri River at St. Louis, Missouri. The Upper Mississippi is divided into two sections:

1. The headwaters, 493 miles (793 km), from the source to Saint Anthony Falls in Minneapolis, Minnesota
2. A navigable channel, formed by a series of man-made lakes between Minneapolis and St. Louis, Missouri, 664 miles (1,069 km)

The source of the Upper Mississippi branch is traditionally accepted as Lake Itasca, 1,475 feet (450 m) above sea level in Itasca State Park in Clearwater County, Minnesota. The name "Itasca" is a combination of the last four letters of the Latin word for truth (veritas) and the first two letters of the Latin word for head (caput). However, the lake is in turn fed by a number of smaller streams, of which one might be selected as the river's ultimate source.

From its origin at Lake Itasca to St. Louis, Missouri, the waterway's flow is moderated by 43 dams. Fourteen of these dams are located above Minneapolis in the headwaters region and serve multiple purposes, including power generation and recreation. The remaining 29 dams, beginning in downtown Minneapolis, all contain locks and were constructed to permit commercial navigation of the upper river. Taken as a whole these 43 dams significantly shape the geography and influence the ecology of the upper river. Beginning just below Saint Paul, Minnesota, and continuing throughout the upper and lower river, the Mississippi is further controlled by thousands of wing dikes that moderate the river's flow in order to maintain an open navigation channel and prevent the river from eroding its banks.

The head of navigation on the Mississippi is the Coon Rapids Dam in Coon Rapids, Minnesota. Before its construction in 1913, steamboats could occasionally go upstream as far as Saint Cloud, depending on river conditions.

The uppermost lock and dam on the Upper Mississippi River is the Upper St. Anthony Falls Lock and Dam in Minneapolis. Above the dam, the river's elevation is 799 feet (244 m). Below the dam, the river's elevation is 750 feet (230 m). This 49-foot (15 m) drop is the largest of all the Mississippi River locks and dams. The origin of the dramatic drop is a waterfall preserved adjacent to the lock under an apron of concrete. Saint Anthony Falls is the only true waterfall on the entire Mississippi River. The water elevation continues to drop steeply as it passes through the gorge carved by the waterfall.

The Upper Mississippi features various natural and artificial lakes, with its widest point being Lake Winnibigoshish, near Grand Rapids, Minnesota, over 7 miles (11 km) across. Also of note is Lake Onalaska, near La Crosse, Wisconsin, over 4 miles (6.4 km) wide. On the other hand, Lake Pepin is natural, formed due to the delta formed by the Chippewa River of Wisconsin as it enters the Upper Mississippi; it is more than 2 miles (3.2 km) wide.

By the time the Upper Mississippi reaches Saint Paul, Minnesota, below Lock and Dam #1, it has dropped more than half its original elevation and is 687 feet (209 m) above sea level. From St. Paul to St. Louis, Missouri, the river elevation falls much more slowly, and is controlled and managed as a series of pools created by 26 locks and dams.

The Upper Mississippi River is joined by the Minnesota River at Fort Snelling in the Twin Cities; the St. Croix River near Prescott, Wisconsin; the Cannon River near Red Wing, Minnesota; the Black River (Mississippi River), La Crosse River, and Root River (Minnesota) in La Crosse, Wisconsin; the Wisconsin River in Prairie du Chien, Wisconsin; the Rock River in the Quad Cities; the Iowa River near Wapello, Iowa; the Skunk River south of Burlington, Iowa; and the Des Moines River in Keokuk, Iowa. Other major tributaries of the Upper Mississippi include the Crow River in Minnesota, the Chippewa River in Wisconsin, the Maquoketa River and the Wapsipinicon River in Iowa, and the Big Muddy River, Illinois River, and the Kaskaskia River in Illinois.

Names and Tributaries of the Mississippi River

The Mississippi River is divided into the Upper Mississippi, the Middle Mississippi, and the Lower Mississippi, with the Upper Mississippi upriver of its confluence with the Missouri River, the Middle Mississippi from there downriver to the Ohio River, and the Lower Mississippi from there downriver to the Gulf of Mexico.