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 beneath like a skeleton.
The precipitation there has not been great enough to destroy the old lines--only enough to mask them.
* There could be a balance of precipitation and still very little snow- or rainfall, or they might be very great.
The "inner gorge" of the Grand Canyon appears to have been cut far more rapidly than the outer one, and at a much later period; were this not the case there would be no inner gorge.