EAINFALL AND DECLIVITY. 
239 
While the precipitation upon the middle and lower levels — .’i,000 to 
6,500 feet above the sea — is on the whole quite small, the transporting 
power of such water as runs into the river is very great. A cubic yard of 
running water in the Plateau Country probably carries several times more 
sediment than the same quantity of water in the Atlantic rivers. This re- 
markable difference is due to the following causes : In the first place the 
soil and comminuted ddbris of the plateaus is not held together by vegeta- 
tion, but lies loosely upon the rocks and taluses, and is easily gathered up 
by rills. But chiefly the slopes are__ always very great, and as the transport- 
ing power increases enormously with the declivity the only limit to the 
quantity of material a given volume of water can carry is reached when 
the mixture becomes so viscous that its own internal friction is great enough 
to seriously retard its rate of motion. In the rivers which drain the Appa- 
lachian region the finer material is supplied in quantity insufficient to load 
the running waters to their full capacity, while the rainfall is very copious 
and the declivities considerable. In the prairies of the Mississippi Valley 
the declivities are small, while the water supply is great and the finer mate- 
rial superabundant. In the plateaus the water supply is small, while the 
declivities are very great and the fine material also is relatively in excess. 
This will sufficiently explain why the spasmodic streams of the plateaus are 
so much more heavily loaded with sediment than those of moister regions. 
THE DECLIVITY OF THE EIVEE. 
The declivity of the Colorado in the Grand Canon next requires our 
attention. Between the junction of the Little Colorado and the Grand 
Wash the absolute altitude of the river bed declines from 2,640 to 1,000 
feet above the sea — a total fall of 1,640 feet. The distance measured along 
the median line of the water surface is 218 miles, giving an average fall of 
7.56 feet per mile. But the rate of descent through the various parts of 
the canon varies considerably. The following table, derived from the meas- 
viuced is as near an approximation to the truth as can be obtained at present, and this statement must 
necessarily he very meager. The great irregularity when dift'ereut years are compared is, I am satisfied, 
unquestionable. 
