RIVER EROSION 161 
burdened condition, is not engaged in deepening its 
channel. The most favorable condition of sediment 
for rapid valley deepening is that of a heavy load, 
always present, but never exceeding the power of the 
river to carry it. Such streams are continuously at 
work carving into the rock. No better illustration 
of this can be found than that remarkable river which . 
has excavated the Colorado Cajon (see p. 170). 
Intermittent Work. A second point of importance 
in valley deepening is the slope of the land, for the 
velocity of flowing water increases with the slope. 
Thus, with a steep incline, the mountain torrents are 
able to carry even large bowlders, and with them to cut 
easily into their beds. 
Even with a given slope, a river may vary in velo- 
city according to the amount of water carried. We 
may see this illustrated in almost any stream of mod- 
erate size. Ordinarily, even though the slope be great, 
the water flows along at a moderate rate; but when 
the floods come it is transformed into a raging torrent, 
when bridges are in danger of destruction (compare 
Frontispiece and Plate 6). The river is then a rapid 
worker, and it is safe to say that'at such times streams 
do more valley deepening in a few hours or days, than 
is done in the remaining months of quiet water. With 
their cutting tools, rivers carve intermittently, now 
rapidly and again slowly or scarcely at all. 
M 
