THE LAWS OF COREASION. 
233 
whole length. Since the stream has the same declivity, and consequent!}^ 
the same velocity at all points, its capacity for corrasion is everywhere the 
same. Its rate of corrasion, however, will depend on the character of its 
bed. Where the rock is hard, corrasion will be less rapid than where it is 
soft, 'and there will result inequalities of grade. But so soon as there is 
inequality of grade there is inequality of velocity and inequality of capacity 
for corrasion; and where hard rocks have produced declivities there the 
capacity for corrasion will be increased. The differentiation will proceed 
until the capacity for corrasion is everywhere proportioned to the resistance 
and no further; that is, until there is equilibrium of action. 
“ In general, we may say that a stream tends to equalize its work in 
all parts of its course. Its power inheres in its fall, and each foot of fall 
has the same power. When its work is to corrade, and the resistance is 
unequal, it concentrates its energy where the resistance is great by crowd- 
ing many feet of descent into a small space, and diffuses it where the resist- 
ance is small, by using but a small fall in a long distance. When its woi’k 
is to transport, the resistance is constant and the fall is evenly distributed by 
a uniform grade. When its work includes both transportation and corra- 
sion, as in the usual case, its grades are somewhat unequal, and the 
inequality is greatest when the load is least.” 
The foregoing analysis is applicable to the Colorado. Among the large 
rivers of the world considered as trunk streams draining large areas its con- 
dition and operations are exceptional, though by no means wholly unique. 
Nearly all large rivers along their lower and middle courses and along con- 
siderable portions of the larger tributaries have reached or nearly approxi- 
mated to that condition of equilibrium of action which Mr. Gilbert speaks 
of, in which the transporting power is nearly adjusted without excess to the 
load to be carried ; and they have little or no tendency either to corrade or 
to deposit. But the Colorado is corrading rapidly, and has doubtless done 
so during a great part of its history. To appreciate fully the conditions 
which determine and sustain this action it is necessary to study them in 
detail. 
