MISSISSIPPI RIVER (Algonquin, Missi Sipi, or "great river"), the central trunk of the great river system draining that part of the United States which lies between the Appalachian Mountains on the east and the Rocky Mountains on the west. Together with over 4o tributaries which are navigable for at least part of their courses it forms one of the great inland naviga tion systems of the world. Over 15,000 m. of waterway are capable of being used for commercial transport purposes. The entire area drained by the river and its tributaries is about 1,240,000 sq.m., or over one-third the area of the United States. Over this drainage area there is an average annual precipitation of 29.8 in., of which about one-fourth ultimately finds its way to the sea via the Mississippi. The total annual discharge at its mouth is estimated at 785,190,000,00o cu.yd., and the total amount of sediment carried into the Gulf annually is about 406,250,000 tons.
The Mississippi river rises in the lake region of northern Minne sota and flows in a southerly direction to the Gulf of Mexico. Its ultimate headspring has been found in Little Elk lake about 2,56o m. from its mouth, though the exact distance varies with a shifting river bed. The river valley may be conveniently divided into the Upper Mississippi and the Lower Mississippi, the con fluence with the Missouri, the longest tributary, being the divid ing point. Like the Missouri and the Ohio, the Upper Mississippi may then be thought of as merely a chief tributary to the Lower Mississippi, and the contributions of the three most important branches may be compared as follows :— to over ioo feet.
As the river flows through its alluvial plain it builds natural embankments or levees along its immediate shore. These natural levees are higher than the remainder of the flood plain, the f all away to the inland averaging 7 ft to the first mile. Often the bed of the river actually lies higher than the surrounding country. The natural levees have been supplemented by artificial levees These three rivers thus represent 63% of the Mississippi's dis charge at the Gulf. Below the mouth of the Ohio the chief tributaries are the Arkansas and Red rivers with extensive drain age basins (187,00o and 93,000 sq.m. respectively) and the St.
Francis and Yazoo with much smaller basins, but with a far heavier rainfall, of which almost 75% finds its way to the Mis sissippi. The Missouri river flows 2,95o m. from the Rocky Mountains before it enters the Mississippi and, if to its length is added the 1,250 m. of the Lower Mississippi, the combination of the two forms the longest river in the world.
From its source at a comparatively slight elevation (1,670 ft. above sea-level) to its mouth the incline of the Mississippi river proper is gentle and almost uniform. Its upper course is through many marshes and lakes, and its valley south to the Falls of St. Anthony at Minneapolis is shallow and young, lying on a bed of glacial deposit through which it has cut but slightly. Entering the driftless area below the Falls of St. Anthony its trench be comes deeper and all the way to Cape Girardeau, Mo., its bed lies 400 to 600 ft. below the level of the surrounding prairies. This valley trench is 2 to 6 m. wide, with a comparatively level floor, and is bordered by abrupt bluffs, mostly wooded, but often crowned above their talus slopes with precipitous limestone and sandstone cliffs which add greatly to the beauty of the region.
This trench was cut in pre-glacial times, and its rock floor, as determined by well borings, was then 10o to 200 ft. below the present river bed and a little steeper in its incline toward the south. The outwash of glacial sand and gravel later filled the trench to, or slightly above, its present level. During the retreat of the ice the Mississippi carried a much greater volume of water than at present. Glacial Lake Agassiz in the valley of the Red river (of the North), with its natural northern drainage blocked by ice flowed southward through the Minnesota river into the Mississippi. Likewise the Great Lakes, their St. Lawrence outlet dammed, drained into the Mississippi; Lake Superior by the St. Croix river, Lake Michigan by the Illinois, and Lake Erie by the Wabash and other rivers.