74 
RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
be referred to in which mid-Tertiary erosion may correspond in time 
with the formation of the Mohave peneplain. In view, however, of 
the great distances between the various localities described and the 
difficulties of correlating periods of erosion, the most that can be said 
is that the available evidence points to the probability that the 
Mohave peneplain was one of great extent and may prove to be a 
valuable means of correlation. 
WATER SUPPLIES. 
SURFACE WATERS. 
PRECIPITATION. 
Little is definitely known of the rainfall in western Arizona. Ac- 
cording to E. C. Murphy, a of the U. S. Geological Survey, the average 
annual precipitation for the region is approximately 5 inches. At 
Needles, Cal., records kept by the Weather Bureau since 1892 show 
an annual mean of 2.47 inches. 
EVAPORATION. 
Evaporation in western Arizona greatly exceeds rainfall. No 
records are available for the area described, but it probably does not 
differ very much from that at Yuma, Ariz., where the annual evapora- 
tion, as given by Murphy, is 82.32 inches. 
STREAMS. 
COLORADO RIVER. 
FIovj. — A gaging station has recently been established at Hardy- 
ville, near old Fort Mohave, for measuring the flow of Colorado 
River. Records, however, are already available for the last four 
years at Yuma, Ariz., which probably give most satisfactorily the 
volume carried by the lower Colorado. 
Discharge of Colorado River at Yuma, Ariz. 
Total in 
acre-feet. 
Mean in 
second-feet. 
1902 
7,960,189 
11,329,032 
10,109,004 
19,710,000 
10,970 
1903 
15,595 
1904 
13,922 
1905 
27,300 
12,277,056 
16,947 
Irrigable lands. — Although a large amount of water is available in 
Colorado River, the irrigable land is limited in amount and confined 
a Personal communication. 
