The Mississippi River is the second-longest river in the United States, with a length of 2,320 miles (3,730 km) from its source in Lake Itasca in Minnesota to its mouth in the Gulf of Mexico.
The Mississippi River is part of the Missouri-Mississippi river system, which is the largest river system in North America and among the largest in the world: by length (3,900 miles (6,300 km)), it is the fourth longest, and by its average discharge of 572,000 cu ft/s (16,200 m³/s), it is the tenth largest.
The name Mississippi is derived from the Ojibwe word misi-ziibi ("Great River") or gichi-ziibi ("Big River").
The Missouri River flows from the confluence of the Jefferson, Madison and Gallatin Rivers and is the longest river in the United States. Taken together, the Jefferson, the Missouri, and the Mississippi form the longest river system in North America. If measured from the source of the Jefferson at Brower's Spring, to the Gulf of Mexico, the length of the Mississippi-Missouri-Jefferson combination is approximately 3,900 miles (6,300 km), making the combination the 4th longest river in the world. The uppermost 207 miles (333 km) of this combined river are called the Jefferson, the lowest 1,352 miles (2,176 km) are part of the Mississippi, and the intervening 2,341 miles (3,767 km) are called the Missouri.
The Arkansas River is the second longest tributary of the Mississippi River. Measured by water volume, the largest of all Mississippi tributaries is the Ohio River.
The widest point of the Mississippi River is Lake Winnibigoshish, near Grand Rapids, Minnesota, at over 7 miles (11 km) across. Also of note is Lake Onalaska, near La Crosse, Wisconsin, where the river is over 4 miles (6.4 km) wide (created by Lock and Dam No. 7) and Lake Pepin at more than 2 miles (3.2 km) wide. However, the first two areas are lakes or reservoirs rather than free flowing water. In other areas where the Mississippi is a flowing river (other than Lake Pepin), it exceeds 1 mile (1.6 km) in width in several places in its lower course.
Beginning with its source at Lake Itasca to Saint Louis Missouri, the Mississippi's flow is moderated by 43 dams. Fourteen of these dams are located above Minneapolis Minnesota 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 Mississippi River runs through 10 states and was used to define portions of these states' borders. The middle of the riverbed at the time the borders were established was the line to define the borders between states. The river has since shifted, but the state borders of Wisconsin, Iowa, Illinois, Missouri, Kentucky, Arkansas, Tennessee, and Mississippi have not changed; they still follow the former bed of the Mississippi River as of their establishment.
The river is divided into the upper Mississippi, from its source south to the Ohio River, and the lower Mississippi, from the Ohio to its mouth near New Orleans.
The source of the Mississippi River is Lake Itasca, 1,475 feet (450 m) above sea level in Itasca State Park located in Clearwater County, Minnesota. The name "Itasca" is a combination of the last four letters of the Latin word for truth and the first two letters of the Latin word for head.
The uppermost lock and dam on the 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. By the time the river 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. From St. Louis to the Ohio River confluence the Mississippi free 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 the Ohio River confluence the Mississippi is 315 feet (96 m) above sea level.
A clear channel is needed for the barges and other vessels that make the main stem Mississippi one of the great commercial waterways of the world. The task of maintaining a navigation channel is the responsibility of the United States Army Corps of Engineers, which was established in 1802. Earlier projects began as early as 1829 to remove snags, close off secondary channels and excavate rocks and sandbars.
Steamboats entered trade in the 1820s, so the period 1830 – 1850 became the golden age of steamboats. As there were few roads or rails in the lands of the Louisiana Purchase, river traffic was an ideal solution. Cotton, timber and food came down the river, as did Appalachia coal. The port of New Orleans boomed as it was the trans-shipment point to deep sea ocean vessels. As a result, the image of the twin stacked, wedding cake Mississippi steamer entered into American mythology. Steamers worked the entire route from the trickles of Montana, to the Ohio river; down the Missouri and Tennessee. To the main channel of the Mississippi. Only the arrival of the railroads in the 1880s did steamboat traffic diminish. Steamboats remained a feature until the 1920s. Most have been superseded by pusher tugs. A few survive as icons—the Delta Queen and the River Queen for instance.
A series of 29 locks and dams on the upper Mississippi, most of which were built in the 1930s, is designed primarily to maintain a 9 feet (2.7 m) deep channel for commercial barge traffic. The lakes formed are also used for recreational boating and fishing. The dams make the river deeper and wider but do not stop it. No flood control is intended. During periods of high flow, the gates, some of which are submersible, are completely opened and the dams simply cease to function. Below St. Louis, the Mississippi is relatively free-flowing, although it is constrained by numerous levees and directed by numerous wing dams.