Mississippi, river, United States: Course and Navigation

Course and Navigation

The Mississippi River rises in small streams that feed Lake Itasca (alt. 1,463 ft/446 m) in N Minnesota and flows generally south to enter the Gulf of Mexico through a huge delta in SE Louisiana. A major economic waterway, the river is navigable from the sediment-free channel maintained through South Pass in the delta to the Falls of St. Anthony in Minneapolis, with canals circumventing the rapids near Rock Island, Ill., and Keokuk, Iowa. For the low-water months of July, August, and September, there is a 45-ft (13.7-m) channel navigable by oceangoing vessels from Head of the Passes to Baton Rouge, La., and a 9-ft (2.7-m) channel from Baton Rouge deep enough for barges and towboats to Minneapolis. The Mississippi connects with the Intracoastal Waterway in the south and with the Great Lakes–St. Lawrence Seaway system in the north by way of the Illinois Waterway.

Along the river's upper course shipping is interrupted by ice from December to March; thick, hazardous fogs frequently settle on the cold waters of the unfrozen sections during warm spells from December to May. In its upper course the river is controlled by numerous dams and falls c.700 ft (210 m) in the 513-mi (826-km) stretch from Lake Itasca to Minneapolis and then falls c.490 ft (150 m) in 856 mi (1,378 km) from Minneapolis to Cairo, Ill. The Mississippi River receives the Missouri River 17 mi (27 km) N of St. Louis and expands to a width of c.3,500 ft (1,070 m); it swells to c.4,500 ft (1,370 m) at Cairo, where it receives the Ohio River. The stretch of the river from the last dam and locks, above St. Louis, to Cairo is also known as the middle Mississippi.

The lower Mississippi meanders in great loops across a broad alluvial plain (25–125 mi/40–201 km wide) that stretches from Cape Girardeau, Mo., to the delta region S of Natchez, Miss. The plain is marked with oxbow lakes and marshes that are remnants of the river's former channels. Natural levees, built up from sediment carried and deposited in times of flood, border the river for much of its length; sediment has also been deposited on the riverbed, so that in places the surface of the Mississippi is above that of the surrounding plain, as evidenced by the St. Francis, Black, Yazoo, and Tensas river basins. Breaks in the levees frequently flood the fertile bottomlands of these and other low-lying areas of the plain.

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