The Mississippi below the mouth of the Ohio is an entirely changed
river. Above that stream, it is similar to most other inland
waterways, having tolerably stable banks and not much change in width.
Below Cairo, however, the deposits forming the banks are composed of
alternate layers of sand and mud or clay, the sand having been
deposited by running water, and the mud in comparatively still water,
so that the sand-layers are readily washed out, thus causing the banks
to cave whenever the current sets against them. Below the influx of
the Ohio, the river traverses alluvial bottom lands of inexhaustible
fertility, and usually stretching to a width of forty miles or more.
These alluvial lands have a general southern slope of about eight
inches to the mile, and stretch five hundred miles to the southward,
the river winding through them in a devious course for eleven hundred
miles, occasionally on the eastern side washing bluffs of one to three
hundred feet. The slope is sufficient to create high velocities in the
current, making a very unstable channel, constantly shifting laterally
and causing the river to develop into a serpentine form, one bend
following another continuously. The immediate river, wherever it may
be at the time, is confined by banks of its own creation, which, like
all sediment-bearing rivers, are highest near the stream itself. Thus
apparently following a low ridge through the bottom lands, the
resistless mass of muddy water sweeps onward with swiftness, eroding
its outer banks in the bends and rebuilding them on the opposite
points, frequently forming islands by its deposits, and as frequently
removing them, as the direction of flow may be modified by the
unending changes in progress. Chief among these changes is the
formation of "cut-offs." Two vast eroding bends covering several miles
of distance gradually approach each other until the water forces a
passage across the narrow neck. As the channel distance between these
bends may have been many miles around, the sudden "cut-off" makes a
cascade of several feet, through which the torrent rushes with a roar
heard far away. The sandy banks dissolve like so much sugar, in a
single day the course of the river is radically changed, and
steamboats pass where a few hours before was cultivated land. The
checking of the current at the upper and lower mouths of the abandoned
channel soon obstructs them with the deposits, and in a few years
forms a crescent-shaped lake, of which there are so many in the
bottoms adjacent to the river. The convex bank in a bend is built up
as rapidly by the deposits as the opposite concave bank washes off, so
that the river does not usually become any wider in the bends on
account of the process. The deepest water is always next to the
concave or wasting bank, where the most current flows. It is not an
unusual sight along this extraordinary river to see an ancient and
well-constructed house hanging over the caving bank, destined
ultimately to drop into the water. It may originally have been a mile
from the river in the centre of an old plantation, but the mighty
current sweeping around and into the bend has worn away the land,
often dissolving it by acres, and as it dropped in, has piled the
sediment on the opposite point, thus steadily moving the river over
without materially changing the width, until it is ready to engulf the
house.
While the great river above the Ohio is generally bordered by
limestone bluffs, making stable conditions, yet below, the Mississippi
flows through a region wholly formed by its own deposits. It is said
the alluvial basin below Cairo was once an estuary of the Gulf of
Mexico, and that it has been raised in level, along with the entire
southern portion of the Continent, about a hundred feet, and then
filled in with the sediment the river carries down. This alluvial
region is sometimes as much as seventy miles wide; and when not
confined to the channel by levees, the natural course of a great
Mississippi flood is to spread entirely over the basin. These floods
will rise fifty feet, and the basin then becomes a great reservoir and
storage-ground for the surplus waters, though the levee system has
much restricted this. It is estimated that the annual discharge of the
Mississippi is twenty-one million millions of cubic feet of water, and
that it carries in a year four hundred millions of tons of solid
material down to the Gulf to be deposited; thus cutting away from its
banks a space equalling ten square miles of territory eighty feet
deep. It takes one-fourth the rainfall of its valley down to the Gulf,
or water equalling a depth of seven or eight inches over its whole
drainage area, and the solid matter annually carried along and
deposited there is equal to a body a mile square and three hundred and
sixty feet high. The flow of the river is from one to six miles an
hour in different stages and sections. The flood periods are in April
and June, the river being above the mid-stage usually from January to
August; and the lowest stage comes generally in October.