Encyclopædia Britannica, Ninth Edition/Mississippi (1.)

MISSISSIPPI. The territory drained by the Mississippi river and its tributaries includes the greater part of the United States of America lying between the Alleghany Mountains on the east and the Rocky Mountains on the west, and has an area (1,244,000 square miles) considerably larger than all central Europe. The central artery through which the drainage of this region passes is called the Mississippi river for about 1300 miles above its mouth. The name is then usurped by a tributary, while the main stream becomes known as the Missouri. From its remote sources in the Rocky Mountains to the Gulf of Mexico the total length of the river is about 4200 miles. The other principal tributaries are the Ohio, the Arkansas, and the Red River, but the Yazoo and the St Francis often make dangerous contributions in seasons of flood.

The tables given below exhibit the hydraulic features of the Mississippi and its principal tributaries.

Below the influx of the Ohio the Mississippi traverses alluvial bottom lands liable to overflow in flood seasons. The soil is of inexhaustible fertility, producing large crops of corn in the northern portion, cotton in the middle district, and sugar, rice, and orange groves near the mouth. These bottom lands, averaging about 40 miles in width, extend from north to south for a distance of 500 miles, having a general southern slope of 8 inches to the mile. The river winds through them in a devious course for 1100 miles, occasionally on the east side washing bluffs from 100 to 300 feet in height, but usually confined by banks of its own creation, which, as with all sediment-bearing rivers of like character, are highest near the stream itself. The general lateral slope towards the foot hills is about 6 inches

in 5000 feet, but the normal fall in the first mile is about 7 feet. Thus apparently following a low ridge through the bottom lands, the tawny sea sweeps onward with great velocity, eroding its banks in the bends and rebuilding them on the points, now forming islands by its deposits, and now removing them as the direction of the flow is modified by the never-ending changes in progress. Chief among such changes is the formation of cut-offs. Two eroding bends gradually approach each other until the water forces a passage across the narrow neck. As the channel distance between these bends may be many miles, a cascade perhaps 5 or 6 feet in height is formed, and the torrent rushes through with a roar audible for miles. The banks dissolve like sugar. In a single day the course of the river is changed, and steamboats pass where a few hours before the plough had been at work. The checking of the current at the upper and lower mouths of the abandoned channel soon obstructs them by deposit, and forms in a few years one of the characteristic crescent lakes which are so marked a feature on the maps.

The total area of the bottom lands is about 32,000 square miles, of which only a narrow strip along the immediate banks of the main river and of its principal bayous and tributaries has even yet been brought under cultivation. A proper system of protection against overflow would throw open 2,500,000 acres of rich sugar land, 7,000,000 acres of the best cotton land in the world, and 1,000,000 acres of corn land of unsurpassed fertility.

Tributaries of the Lower Mississippi.

The Lower Mississippi.

{|align="center" cellpadding="0" cellspacing="0" rules="cols" border="1"
 * align="center"|River.
 * align="center"|Distance from Mouth.
 * align="center"|High Water Elevation above Sea.
 * align="center"|Fall per Mile.
 * align="center"|Width between Banks.
 * align="center"|Least Low Water Depth upon the Bars.
 * align="center"|Range between High and Low Water.
 * align="center"|Area of Cross Section at High Water.
 * align="center" width="20%"|Remarks.
 * align="center"|Miles.
 * align="center"|Feet.
 * align="center"|Feet.
 * align="center"|Feet.
 * align="center"|Feet.
 * align="center"|Feet.
 * align="center"|Square Feet.
 * align="left"| Mouth of Missouri
 * align="right"|1,286
 * align="right"|416.0
 * align="center"|...
 * align="center"|...
 * rowspan="3"|
 * align="center"|...
 * align="center"|...
 * rowspan="14"|
 * align="left"| St Louis
 * align="right"|1,270
 * align="right"|408.0
 * align="center"|0.500
 * align="center"|...
 * align="right"|37.0
 * align="center"|...
 * align="left"| Cairo
 * align="right"|1,097
 * align="right"|322.0
 * align="center"|0.497
 * rowspan="3"|
 * align="right"|51.0
 * rowspan="3"|
 * align="left"| Columbus
 * align="right"|1,076
 * align="right"|310.0
 * align="center"|0.571
 * rowspan="2"|
 * align="right"|47.0
 * align="left"| Memphis
 * align="right"|872
 * align="right"|221.0
 * align="center"|0.436
 * align="right"|40.0
 * align="left"| Gaines landing
 * align="right"|647
 * align="right"|149.0
 * align="center"|0.320
 * rowspan="3"|
 * rowspan="3"|
 * align="center"|...
 * rowspan="3"|
 * align="left"| Natchez
 * align="right"|378
 * align="right"|66.0
 * align="center"|0.309
 * align="right"|51.0
 * align="left"| Red River landing
 * align="right"|316
 * align="right"|49.5
 * align="center"|0.266
 * align="right"|44.3
 * align="left"| Baton Rouge
 * align="right"|245
 * align="right"|33.9
 * align="center"|0.220
 * rowspan="2"|
 * align="center"|...
 * align="right"|31.1
 * rowspan="2"|
 * align="left"| Donaldsonville
 * align="right"|193
 * align="right"|25.8
 * align="center"|0.156
 * align="center"|...
 * align="right"|24.3
 * align="left"| Carrollton
 * align="right"|121
 * align="right"|15.2
 * align="center"|0.147
 * rowspan="3"|
 * align="center"|...
 * align="right"|14.4
 * rowspan="3"|
 * align="left"| Baton Rouge
 * align="right"|245
 * align="right"|33.9
 * align="center"|0.220
 * rowspan="2"|
 * align="center"|...
 * align="right"|31.1
 * rowspan="2"|
 * align="left"| Donaldsonville
 * align="right"|193
 * align="right"|25.8
 * align="center"|0.156
 * align="center"|...
 * align="right"|24.3
 * align="left"| Carrollton
 * align="right"|121
 * align="right"|15.2
 * align="center"|0.147
 * rowspan="3"|
 * align="center"|...
 * align="right"|14.4
 * rowspan="3"|
 * align="right"|14.4
 * rowspan="3"|


 * align="left"| Fort St Philip
 * align="right"|37
 * align="right"|5.2
 * align="center"|0.119
 * align="center"|...
 * align="right"|4.5
 * align="left"| Head of Passes
 * align="right"|17
 * align="right"|2.9
 * align="center"|0.115
 * align="center"|...
 * align="right"|2.3
 * align="left"| Gulf
 * align="right"|0
 * align="right"|0.0
 * align="center"|0.171
 * align="center"|...
 * align="center"|...
 * align="right"|0.0
 * align="center"|...
 * }
 * align="right"|0.0
 * align="center"|...
 * }

The work of embankment began in 1717, when the engineer De la Tour erected a dyke or levee 1 mile long to protect the infant city of New Orleans from overflow. Progress at first was slow. In 1770 the settlements extended only 30 miles above and 20 miles below New Orleans; but by 1828 the levees, although quite insufficient in dimensions, had become continuous nearly to the mouth of Red River. In 1850 a great impulse was given to systematic embankment by the U.S. Government, which gave over to the several States all unsold swamp and overflowed lands within their limits to provide a fund for reclaiming the districts liable to inundation. The action

resulting from this caused alarm in Louisiana, for the great bottom lands above were believed to act as reservoirs to receive the highest flood wave; and it was imagined that if they were closed by levees the lower country would be overwhelmed whenever the river in flood rose above its natural banks. The aid of the Government was invoked, and Congress immediately ordered the necessary investigations and surveys. This work was placed in charge of Captain (now General) Humphreys, and an elaborate report covering the results of ten years of investigation was published just after the outbreak of the civil war in 1861. The second of the tables given above, and indeed most of the physical facts respecting the river, are quoted from this standard authority.

To understand the figures of the table it should be noted that at the mouth of Red River, 316 miles above the passes, the water surface at the lowest stage is only 5$78/100$ feet above the level of the Gulf, where the mean tidal oscillation is about 1$15/100$ feet. The river channel in this section is therefore a freshwater lake, nearly without islands, 2600 feet wide and 100 feet deep along the deepest line. At the flood stage the surface rises 50 feet at the mouth of Red River, but of course retains its level at the Gulf, thus giving the head necessary to force forward the increased volume of discharge. Above the mouth of Red River the case is essentially different. The width increases and the depth decreases; islands become numerous; the oscillation between high and low water varies but little from 50 feet until the mouth of the Ohio is reached—a distance of about 800 miles. Hence the general slope in long distances is here nearly the same at all stages, and the discharge is regulated by the varying resistances of cross section, and by local changes in slope due to the passage of flood waves contributed by the different tributaries. The effect of these different physical conditions appears in the comparative volumes which pass through the channel. At New Orleans the maximum discharge hardly reaches 1,200,000 cubic feet per second, and a rising river at high stages carries only about 100,000 cubic feet per second more than when falling at the same absolute level; while just below the mouth of the Ohio the maximum flood volume reaches 1,400,000 cubic feet per second, and at some stages a rising river may carry one-third more water than when falling at the same absolute level.

The percentage of sedimentary matter carried in suspension by the water varies greatly at different times, but is certainly not dependent upon the stage above low water. It is chiefly determined by the tributary whence the water proceeds, but is also influenced by the caving of the banks, which is always excessive when the river is rapidly falling after the spring flood. In long periods the sedimentary matter is to the water by weight nearly as 1 to 1500, and by bulk as 1 to 2900. The amount held in suspension and annually contributed to the Gulf constitutes a prism 1 mile square and 263 feet high. In addition to this amount a large volume, estimated at 1 mile square and 27 feet high annually, is pushed by the current along the bottom and thus transported to the Gulf.

The mean annual succession of stages for long periods is quite uniform, but so many exceptions are noted that no definite prediction can safely be made for any particular epoch. The river is usually lowest in October. It rises rapidly until checked by the freezing of the northern tributaries. It begins to rise again in February, and attains its highest point about the 1st of April. After falling a few feet it again rises until, early in June, it attains nearly the same level as before. After this it rapidly recedes to low-water mark. As a rule the river is above mid-stage from January to August inclusive, and below that level for the remainder of the year.

It has been established by measurement and observation that the great bottom lands above Red River before the construction of their levees did not serve as reservoirs to diminish the maximum wave which passed through Louisiana in great flood seasons. They had already become filled by local rains and by water escaping into them from the Mississippi through numerous bayous, so that at the date of highest water the discharge into the river near their southern borders was fully equal to the volume which the wave had lost in passing along their fronts.

In fine, the investigations between 1850 and 1860 established that no diversion of tributaries was possible; that no reservoirs artificially constructed could keep back the spring freshets which caused the floods; that the making of cut-offs, which had sometimes been advocated as a measure of relief, so far from being beneficial, was in the highest degree injurious; that, while outlets within proper limits were theoretically advantageous, they were impracticable from the lack of suitable sites; and, finally, that levees properly constructed and judiciously placed would afford protection to the entire alluvial region.