e beds must be held against the wear of the
stream, and much of the opposite banks defended at critical points. The
works having in view this conservative object may be generally
designated works of revetment; and these also will be largely of
brushwood, woven in continuous carpets, or twined into wire-netting.
This veneering process has been successfully employed on the Missouri
River; and in some cases they have so covered themselves with sediments,
and have become so overgrown with willows, that they may be regarded as
permanent. In securing these mats rubble-stone is to be used in small
quantities, and in some instances the dressed slope between high and low
river will have to be more or less paved with stone.

Any one who has been on the Rhine will have observed operations not
unlike those to which we have just referred; and, indeed, most of the
rivers of Europe flowing among their own alluvia have required similar
treatment in the interest of navigation and agriculture.

The levee is the crowning work of bank revetment, although not
necessarily in immediate connection. It may be set back a short
distance from the revetted bank; but it is, in effect, the requisite
parapet. The flood river and the low river cannot be brought into
register, and compelled to unite in the excavation of a single permanent
channel, without a complete control of all the stages; and even the
abnormal rise must be provided against, because this would endanger the
levee, and once in force behind the works of revetment would tear them
also away.

Under the general principle that the local slope of a river is the
result and measure of the resistance of its bed, it is evident that a
narrow and deep stream should have less slope, because it has less
frictional surface in proportion to capacity; i.e., less perimeter in
proportion to area of cross section. The ultimate effect of levees and
revetments confining the floods and bringing all the stages of the river
into register is to deepen the channel and let down the slope. The first
effect of the levees is to raise the surface; but this, by inducing
greater velocity of flow, inevitably causes an enlargement of section,
and if this enlargement is prevented from being made at the expense of
the banks, the bottom must give way and the form of the waterway be so
improved as to admit this flow with less rise. The actual experience
with levees upon the Mississippi River, with no attempt to hold the