alley (exaggerated)

_R R_, rocky basis of valley; _A A_, sedimentary strata; _B_, ordinary
level of river; _C_, flood level.]

The Reno, the most dangerous of all the Apennine rivers, is in some
places as much as 30 feet above the adjoining country. Rivers under such
conditions, when not interfered with by Man, sooner or later break
through their banks, and leaving their former bed, take a new course
along the lowest part of their valley, which again they gradually raise
above the rest. Hence, unless they are kept in their own channels by
human agency, such rivers are continually changing their course.

If we imagine a river running down a regularly inclined plane in a more
or less straight line; any inequality or obstruction would produce an
oscillation, which when once started would go on increasing until the
force of gravity drawing the water in a straight line downwards equals
that of the force tending to divert its course. Hence the radius of the
curves will follow a regular law depending on the volume of water and
the angle of inclination of the bed. If the fall is 10 feet per mile and
the soil homogeneous, the curves would be so much extended that the
course would appear almost straight. With a fall of 1 foot per mile the
length of the curve is, according to Fergusson, about six times the
width of the river, so that a river 1000 feet wide would oscillate once
in 6000 feet. This is an important consideration, and much labour has
been lost in trying to prevent rivers from following their natural law
of oscillation. But rivers are very true to their own laws, and a change
at any part is continued both upwards and downwards, so that a new
oscillation in any place cuts its way through the whole plain of the
river both above and below.

The curves of the Mississippi are, for instance, for a considerable part
of its course so regular that they are said to have been used by the
Indians as a measure of distance.

If the country is flat a river gradually raises the level on each side,
the water which overflows during floods being retarded by reeds, bushes,
trees, and a thousand other obstacles, gradually deposits the solid
matter which it contains, and thus raising the surface, becomes at
length suspended, as it were, above the general level. When this
elevation has reached a certain point, the river during some flood
bursts its banks, and deserting its old bed takes a new course along the
lowest accessible level. Th