26 BULLETIN", 1430, U. S. DEPARTMENT OF AGRICULTURE 
altitude of about 8,000 feet, soon drops into a sandy valley where it 
becomes a river of flowing sand. In ordinary stages it is very wide 
but very shallow rippling over the sand in tawny waves." Only an 
insignificant portion of the basins of these streams is really forested, 
so little that forests play a negligible role in affecting their regimen, 
which is determined by the heavy, irregular rains, the unconsolidated 
soils through which they flow, and the high summer temperature on 
the Plains. 
At Yuma, below all of the important tributaries, the silt content 
of the Colorado River is estimated to average 7,000 parts per million 
and the total solid burden to amount yearly to 160,000,000 tons, of 
which less than 10,000,000 tons comes from the northern sources 
covering one-half of the basin and furnishing three-fourths of the 
discharge. Since the silt content is diluted by the smaller solid 
burden and larger volume of the northern sources " it shows that the 
lower Colorado River derives most of its enormous load of silt from 
the drainage area which includes the San Juan, the Paria, the Little 
Colorado, the Virgin, and the Gila Rivers," and that this silt comes 
from the region of unconsolidated soils with the scant cover of vege- 
tation, limited precipitation consisting largely of rain which often 
falls in heavy showers. A rain of about 4 inches on September 17 
and 18, 1923, over a portion of the basin of the Little Colorado caused 
an increase in discharge of the main Colorado from a normal of 
10,000 feet per second to 110,000 feet per second. Except in the low 
mountains, which are only in part forest covered, the rainfall is too 
scant to support any general vegetative ground cover sufficiently 
dense to afford protection to the soil. Consequently erosion is active 
even where the rainfall is slight. 
The potential horsepower of the Colorado River has been placed 
at 7,000,000, to be secured largely through the employment of 13 
power sites and storage reservoirs on the main stream svnd the moun- 
tain-fed tributaries. The proposed Boulder Canyon Dam, at which 
the largest development would be obtained, would be 600 feet high 
and would impound 32,000,000 acre-feet of water, developing about 
600,000 constant horsepower and 1,500,000 horsepower three-fourths 
of the time. The water, heavily laden with silt, would be desilted 
by sedimentation in the reservoir. (See footnote 11, p. 6.) Much 
of the water used for power purposes would also be available for 
irrigation, and the city of Los Angeles plans to use the desilted 
water as a source of municipal supply. It is estimated that the 
storage capacity of this reservoir would as a result of silting be 
largely destroyed within 150 years. Since the silt burden of the 
river at this point is often more than 2 per cent, the rate of shrinkage 
in storage capacity will probably be far more rapid than indicated 
by this estimate. 
STREAMS OF THE SACRAMENTO AND SAN JOAQUIN BASINS 
The streams of the Sacramento and San Joaquin Basins are subject 
to violent early spring freshets, originating in the heavy rains in the 
mountains ; and their high silt burden has damaged the navigable es- 
tuary channels. The greater portion of the silt of these streams was 
formerly the result of hydraulic mining and even now much silt is de- 
