early driven away from the main
river. A multitude of such crescent-shaped lakes, scattered far and wide
over the alluvial plain, the greater number of them to the west, but
some of them also eastward of the Mississippi, bear testimony of the
extensive wanderings of the great stream in former ages. For the last
two hundred miles above its mouth the course of the river is much less
winding than above, there being only in the whole of that distance one
great curve, that called the "English Turn." This great straightness of
the stream is ascribed by Mr. Forshey to the superior tenacity of the
banks, which are more clayey in this region.

[Illustration: Fig. 24.

Section of channel, bank, levees (_a_ and _b_), and swamps of
Mississippi river.]

The Mississippi has been incorrectly described by some of the earlier
geographers, as a river running along the top of a long hill, or mound
in a plain. In reality it runs in a valley, from 100 to 200 or more feet
in depth, as _a_, _c_, _b_, fig. 24, its banks forming long strips of
land parallel to the course of the main stream, and to the swamps _g_,
_f_, and _d_, _e_, lying on each side. These extensive morasses, which
are commonly well-wooded, though often submerged for months
continuously, are rarely more than fifteen feet below the summit level
of the banks. The banks themselves are occasionally overflowed, but are
usually above water for a breadth of about two miles. They follow all
the curves of the great river, and near New Orleans are raised
artificially by embankments (or levees), _a b_, fig. 24, through which
the river when swollen sometimes cuts a deep channel (or crevasse),
inundating the adjoining low lands and swamps, and not sparing the lower
streets of the great city.

The cause of the uniform upward slope of the river-bank above the
adjoining alluvial plain is this: when the waters charged with sediment
pass over the banks in the flood season, their velocity is checked among
the herbage and reeds, and they throw down at once the coarser and more
sandy matter with which they are charged. But the fine particles of mud
are carried farther on, so that at the distance of about two miles, a
thin film of fine clay only subsides, forming a stiff unctuous black
soil, which gradually envelops the base of trees growing on the borders
of the swamps.

_Waste of the banks._--It has been said of a mountain torrent, that "it
lays down what it will remove, and removes what it