Mississippi-Missouri. The Mississippi River (Algonkin Missi Sipi, 'Great River'), the largest river of North America, is, with its tributaries, wholly within the boundaries of the United States. It drains most of the territory between the Rocky and Alleghany Mountains, embracing an area of 1,257,545 sq. m., or more than two-fifths of the United States. Besides the Missouri, Ohio, Red River, and Arkansas, there are forty-one other tributaries navigable. The total length of the Mississippi is 2960 miles, of which 2161 are navigable; but the Missouri affluent is longer than the Upper Mississippi, and with the lower river gives a total of 4200 miles. The total navigable waters amount to 16,090 miles.

The source of the Mississippi is Lake Itasca (7 by 3 miles) in the north-west central part of Minnesota, which has, however, several feeders, the principal being Elk or Glazier Lake. The remotest springs of Itasca rise in 47° 34' N. lat. and 95° 20' W. long., and are 1680 feet above sea-level. As it issues from this lake the Mississippi is about 12 feet wide and 18 inches deep. Through pine-forests and swamps for hundreds of miles it winds from lake to lake, with frequent rapids and picturesque falls, until, 400 yards wide, at Minneapolis it plunges over the Falls of St Anthony - the head of river-navigation. After receiving the St Croix, the Mississippi becomes the boundary between the states of Minnesota, Iowa, Missouri, Arkansas, and Louisiana on the right, and Wisconsin, Illinois, Kentucky, Tennessee, and Mississippi on the left. On the Wisconsin boundary the river expands into Lake Pepin, and thereafter, fully a mile wide, flows between bluffs 200 and 300 feet high, and sometimes through dense forests. At Rock Island there are rapids with 22 feet of fall, and 125 miles farther down are the Des Moines rapids with 24 feet of fall. Around these obstructions to navigation the United States government has constructed ship-canals. For several miles after the entrance of the turbid Missouri the diverse waters refuse to mingle, the Missouri's muddy tribute taking the right bank and the Upper Mississippi's clear stream the left. From the mouth of the Ohio the trough of the Mississippi is about 1490 yards wide, but as it approaches the Red it is narrowed to 1000, and at New Orleans to 830. The usual depth of the channel southward from the Ohio is from 75 to 100 feet, and its surface is sometimes higher than the country beyond its banks ; from the Missouri to the Gulf the Mississippi rolls in serpentine course through vast alluvial tracts or 'bottoms,' whose width varies from 30 to 150 miles. Though of unsurpassed fertility, scarcely one-tenth of these lands is cultivated owing to the dangers of the annual overflow. The melting of the ice and snow in the upper basin swells the lower current from March to June. Levees or embankments, largely built by the government, now extend for more than 1600 miles. Below the Red River the waters are discharged through numerous ' bayous' into the Gulf of Mexico. The main channel runs south-eastward, and finally divides into five or six passes - the principal, the South Pass. The vast deposits and the constant changes caused by floods tend to embarrass the entrance to the great river. To keep an open channel, Captain Eads made (1875-79) an admirably successful system of jetties at the South Pass, which has secured a depth exceeding 30 feet. The mouth of the Mississippi is essentially tideless. The principal cities on the great river are Minneapolis, St Paul, La Crosse, Dubuque, Keokuk, Quincy, Hannibal, St Louis, Memphis, and New Orleans, at several of which the river is crossed by railway bridges (at St Louis by two).