flood plain the river meanders;
its current, heavily loaded with sediment, swings from one side to the
other of the channel, building up here, wearing away there, and
straightening its course when the curves become so sharp that their
sides meet. Then the current breaks through the thin wall, and a bayou
of still water is left behind.

Below Baton Rouge the Mississippi breaks into many mouths, that spread
and carry the water of the great river into the Gulf of Mexico. The Nile
delta is triangular, like delta, [Greek: D], the fourth letter of the
Greek alphabet; but the Mississippi's delta is very irregular. The main
mouth of the river flows fifty miles out into the Gulf between
mud-banks, narrow and low. At the tip it branches into several streams.

From the mouth of the Ohio to the Gulf, the Mississippi flood plain
covers 30,000 square miles. Over this area, sediment to an average depth
of fifty feet has been laid down. In earlier times the river flooded
this whole area, when freshets swelled its tributaries in the spring.
The flood plain then became a sea, in the middle of which the river's
current flowed swiftly. The slow-flowing water on each side of the main
current let go of its burden of sediment and formed a double ridge.
Between these two natural walls the main river flowed. When its level
fell, two side streams, running parallel with the main river drained the
flood plains on each side into the main tributaries to right and left.
These natural walls deposited when the river was in flood are called
_levees_. Each heavy flood builds them higher, and the bed of the stream
rises by deposits of sediment. So it happens that the level of the river
bed is higher than the level of its flood plain.

This is an interesting fact in geology. But the people who have taken
possession of the rich flood plain of the Mississippi River, who have
built their homes there, drained and cultivated the land, and built
cities and towns on the areas reclaimed from swamps, recognize the
elevation of the river bed as the greatest danger that threatens them.
Suppose a flood should come. Even if it does not overflow the levees, it
may break through the natural banks and thus overflow the cities and the
farm lands to left and right.

Instead of living in constant fear of such a calamity, the people of the
Mississippi flood plain have sought safety by making artificial levees,
to make floods impossible. These are built upon the natural lev