INVARIABLY IN THE HIGHEST REGIONS and at a long distance from their
mouths, so that the flood waters have many miles of opportunity to
run a race with the comparatively feeble erosive forces of desert
lands. The main stream-courses are thus in the lower arid regions and
in sedimentary formations, while their water-supply comes from far
away. The deepest gorges, therefore, will be found where the rainfall
is least, unless diminishing altitude interferes. Thus the greatest
gorge of the whole basin, the Grand Canyon, is the one farthest from
the sources of supply, and in the driest area, but one, of the whole
drainage system. It ends abruptly with the termination of the high
arid plateau which made it possible, but had this plateau extended
farther, the Grand Canyon would also have extended a similar
distance. It is plain then that the cutting of these canyons depends
on the amount of water (snow is included) which may fall in the high
mountains, the canyons themselves being in the drier districts. It is
also clear that if, by some chance, the precipitation of the high
sources should increase, the corrasion of the stream-beds in the
canyons would likewise increase and outrun with still greater ease
the erosion of their immediate surroundings. On the other hand, if
the precipitation in the arid surroundings should increase, the
wearing down of the side walls would for a time--till covered by
debris and vegetation--go on more rapidly till, instead of Canyons of
the Colorado River type, there would be deep, sharp valleys, or wide
valleys, according to the amount of difference between the
precipitation of the low lands and the high. Where the two were
nearly the same, that is, a balance of precipitation,* the slopes
might be rounded and verdure-clad, though this would depend on the
AMOUNT of precipitation. On lower Snake River a change seems to be
going on. The former canyon-cliffs are covered by debris and
vegetation, but in places the old dry cliff-lines can be discerned