ree or four hundred
feet, the entangled stems and branches making it difficult for a boat
to pass from one side of the river to the other.

[Illustration: Fig. 11.--Oxbows and cut-off. Showing the changes in
the course of a river in its alluvial plain.]

When the curves of a river have been developed to a certain point (see
Fig. 11), when they have attained what is called the "oxbow" form, it
often happens that the stream breaks through the isthmus which
connects one of the peninsulas with the mainland. Where, as is not
infrequently the case, the bend has a length of ten miles or more, the
water just above and below the new-made opening is apt to differ in
height by some feet. Plunging down the declivity, the stream, flowing
with great velocity, soon enlarges the channel so that its whole tide
may take the easier way. When this result is accomplished, the old
curve is deserted, sand bars are formed across their mouths, which may
gradually grow to broad alluvial plains, so that the long-surviving,
crescent-shaped lake, the remnant of the river bed, may be seen far
from the present course of the ever-changing stream. Gradually the
accumulations of vegetable matter and the silt brought in by floods
efface this moat or oxbow cut-off, as it is so commonly termed.

As soon as the river breaks through the neck of a peninsula in the
manner above described, the current of the stream becomes much swifter
for many miles below and above the opening. Slowly, however, the
slopes are rearranged throughout its whole course, yet for a time the
stream near the seat of the change becomes straighter than before, and
this for the reason that its swifter current is better able to dispose
of the _debris_ which is supplied to it. The effect of a change in the
current produced by such new channels as we have described as forming
across the isthmuses of bends is to perturb the course of the stream
in all its subsequent downward length. Thus an oxbow cut-off formed
near the junction of the Ohio and Mississippi may tend more or less to
alter the swings of the Mississippi all the way to the Gulf of Mexico.

Although the swayings of the streams to and fro in their alluvial
plains will give the reader some idea as to the struggle which the
greater rivers have with the _debris_ which is committed to them, the
full measure of the work and its consequences can only be appreciated
by those who have studied the phenomena on the ground. A river such
as