VOLUME OP PLOW OP THE COLOEADO. 
235 
the Grand Canon — a distance of about 500 miles— the Colorado has only 
two ti’ibutaries, which bring into it considerable bodies of water, and which 
at the same time run in the open air, as distinguished from subterranean 
streams. These are the San Juan and Little Colorado. The volumes of 
these rivers at their confluences are not accurately known, but during the 
greater portion of the year they are quite notable. They also bring great 
quantities of mud, sand, and gravel. Both enter the left bank of the river. 
All the other living streams are very small — in fact, mere rills — except at 
the times of irregular floods. On the other hand, the evaporation of the 
water in this part of the river in summer time must be very great.* The 
dryness of the air is extreme and the heat intense. In June, July, August, 
and September the midday temperatiu-e of the air is seldom below 90°, and 
often exceeds 110° Fahr., while the relative humidity is only from 0.3 to 0.4 
of saturation. Still the river probably receives more water from the springs 
in the canons and from the two larger tributaries than it loses by evapora- 
tion, and though the excess of this gain over the loss cannot be accurately 
valued I believe it to be considerable. Much importance is here attached 
to the fact that this increase in the volume of the stream in the canons con- 
sists largely of water wholly free from sediment. It means an increase of 
transporting power in the river without an equivalent increase in the amount 
of material to be transported. 
The quantity of water which the Colorado carries varies, of course, 
enormously from season to season throughout the year. A rough estimate 
may be made of its volume near its lowest stage. In the year 1877 Prof. 
A. H. Thompson gauged the flow of the Green and Grand Rivers near their 
junction, in the latter part of September, and found 4,400 cubic feet per 
second in the Green and 4,860 cubic feet per second in the Grand. The 
sum of the two, 9.260 cubic feet per second, is no doubt less than the flow 
in the Grand and Marble Canons. The year was, so far as can be judged, 
very nearly an average one. At the time the measurement was made the 
river was very near its minimum. 
* A water surface 500 miles in length, with an average width of 400 feet and an average evapora- 
tion of 0.4 inch per day from every square foot of .surface, would lose hy evaporation 407.4 cubic feet of 
water every second. This is equivalent to a stream of water 45 feet wide with an average depth of 3 
feet flowing 3 feet per second. The rate of evaporation of 0.4 inch per twenty-four hours is, I believe, 
a low estimate for the Colorado in the great chain of canons during the hot mouths. 
