FOREST COVER IN PROTECTING RESERVOIRS 25 
COLORADO RIVER AND GREAT BASIN 
Two fundamentally different conditions are exemplified in the 
streams which drain the Colorado River basin. One condition is 
represented at the headwaters of the northern streams, the other 
in all the remaining portions of the basin. 
The headwaters of the northern streams, in southwestern Wyo- 
ming and in Colorado, have their sources within the forested moun- 
tain slopes. They are partly fed by melting snows and are largely 
clear until they enter the Plains. The precipitation in the moun- 
tains, much of it in the form of snow, varies from 16 to 40 inches a 
year; on the Plains the yearly average rarely rises above 15 inches, 
being lowest in summer and highest between December and April. 
The soils of the Plains are of many origins and vary in character 
but are prevailingly unconsolidated. The rainfall is too scant to 
support more than a growth of brush and chaparral or in places open 
woodland. The flood seasons of the northern streams are in the 
spring, in June, and in the fall. The spring and fall floods are due 
to rain, partly on the Plains; but the June floods, which are the 
highest, come from melting snows in the mountains. The early 
spring and autumn floods which result from rains, especially on 
the Plains with their scanty protection of vegetative cover, produce 
the greatest turbidity. The June floods, which are the highest, are not 
marked apparently by increase in silt burden. The rather limited 
record at Topock shows a stream of " only normal irregularity " 
approaching even the Columbia in evenness of flow. The relation of 
minimum run-off (January 4, 1925) to maximum (June 22, 1921) 
is as 1 to 92. 
Forests are important in protecting these mountain head streams, 
in holding snow banks, in shading them and in retarding their 
melting. The absence of forests or even of adequate soil cover on 
the Plains, where the rainfall is insufficient to support such a growth, 
accounts for the high silt burden acquired by these streams in their 
passage through the Plains notwithstanding the prevailingly gentle 
topography. 
The streams of the Great Basin and the southern tributaries of 
the Colorado River beginning with the San Juan have their sources 
either within the Plains or in low mountain ranges in Arizona, New 
Mexico, southern Colorado, Utah, and Nevada. Not only is the 
total precipitation which feeds these streams less, rarely rising above 
20 inches and then only along the higher mountains, and with an 
average rainfall as low as 10 inches over much of the Plains and 
desert country, but very little of it is in the form of snow. Summer 
rains are the rule. Violent thunderstorms occur frequently, accom- 
panied generally by high winds and sometimes by very concentrated 
precipitation. Great clouds of dust are usually carried by the wind 
in front of the storm. These heavy rains may alternate with long- 
periods of drought. The Plains consequently have a scant vegeta- 
tion of brush or chaparral and the areas of lowest rainfall are deserts 
with cactus as characteristic shrubby vegetation and very little grass 
cover. High temperatures over the lower-lying lands make a high 
evaporation factor and add to the irregularity of the run-off result- 
ing from the sometimes concentrated rainfall. The Virgin River, 
which is a characteristic stream of this group, though " rising at an 
