The Mississippi River is the second longest river in the United States, with a length of 2,340 miles (3,766 km) from its source in Lake Itasca (Minnesota) to its mouth in Gulf of Mexico (The longest is its tributary the Missouri River measuring 2565 miles). The Mississippi River is part of the Jefferson-Missouri-Mississippi river system, which is the largest river system in North America and among the largest in the world: by length (6,275 km or 3,900 miles), it is the fourth longest, and by average discharge (16,200 m³/s), it is the tenth largest. The longest of the many long Mississippi tributaries is the Missouri River with the Arkansas River as second longest. Measured by water volume, the largest of all Mississippi tributaries is the Ohio River. The name Mississippi is derived from the Ojibwe word misi-ziibi meaning 'great river' (gichi-ziibi 'big river' at its headwaters).
The widest point of the Mississippi River is located in Clinton, Iowa, where the river is nearly 4 miles wide.
From its source at Lake Itasca, 1,475 feet (450 m) above sea level in Itasca State Park located in Clearwater County, Minnesota, the river falls to 801 feet (244 m) prior to St. Anthony Falls in Minneapolis. There it drops to 725 feet (220 m), creating the only waterfall along the river's course. The Mississippi is joined by the Minnesota River in Minneapolis, Minnesota, the Wisconsin River in Prairie du Chien, Wisconsin, the Des Moines River in Keokuk, Iowa, the Illinois River and the Missouri River near St. Louis, Missouri, and by the Ohio River at Cairo, Illinois. The Arkansas River joins the Mississippi in the state of Arkansas. The Atchafalaya River in Louisiana is a major distributary of the Mississippi.
The Mississippi drains most of the area between the Rocky Mountains and the Appalachian Mountains, except for the areas drained by Hudson Bay via the Red River of the North, the Great Lakes and the Rio Grande. It runs through two states — Minnesota and Louisiana — and was used to define the borders of eight states. The river has since shifted, but the state borders of Wisconsin, Iowa, Illinois, Missouri, Kentucky, Arkansas, Tennessee, and Mississippi have not changed. The river empties into the Gulf of Mexico about 100 miles (160 km) downstream from New Orleans. Measurements of the length of the Mississippi from Lake Itasca to the Gulf of Mexico vary, but the EPA's number is 2,320 miles (3,733 km). The retention time from Lake Itasca to the Gulf is about 90 days.
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 upper Mississippi is further divided into three sections: the headwaters, from the source to Saint Anthony Falls; a series of man-made lakes between Minneapolis and St. Louis, Missouri; and the middle Mississippi, a relatively free-flowing river downstream of the confluence with the Missouri River at St. Louis.
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 foot (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.
Through a natural process known as delta switching the lower Mississippi River has shifted its final course to the ocean every thousand years or so. This occurs because the deposits of silt and sediment begin to clog its channel, raising the river's level and causing it to eventually find a steeper, more direct route to the Gulf of Mexico. The abandoned distributary diminishes in volume and forms what are known as bayous. This process has, over the past 5,000 years, caused the coastline of south Louisiana to advance toward the Gulf from 15 to 50 miles (25 to 80 km).
U.S. government scientists determined in the 1950s that the Mississippi River was starting to switch to the Atchafalaya River channel because of its much steeper path to the Gulf of Mexico. Eventually the Atchafalaya River would capture the Mississippi River and become its main channel to the Gulf of Mexico, leaving New Orleans on a side channel. As a result, the U.S. Congress authorized a project called the Old River Control Structure, which has prevented the Mississippi River from leaving its current channel that drains into the Gulf via New Orleans. Because of the large scale of high energy water flow through the Old River Control Structure threatening to damage the structure, an auxiliary flow control station was built adjacent to the standing control station. This US$300 million project was completed in 1986 by the Army Corps Of Engineers.
- General Information about the Mississippi River. Mississippi National River and Recreation Area. National Park Service (2004). Retrieved on 2006-07-15.
- Mississippi River. USGS: Status and trends of the nation's biological resources. Retrieved on 2007-02-03.
- U.S. Waterway System Facts, December 2005 (PDF). USACE Navigation Data Center. Retrieved on 2006-04-27.