FOREST COVER IN PROTECTING RESERVOIRS 27 
rived from old tailings which are still in transit. California, like 
Colorado, has laws prohibiting contaminating streams with tailings 
and the necessary means have been taken to control this through set- 
tling beds. In spite of this control measure, however, the Sacramento 
Kiver bears annually more than 2,250,000 tons of solid burden. 
Much of this is acquired in the foothills and plains, though consid- 
erable erosion takes place in the higher mountains and on the de- 
forested region above Redding. Except at high altitudes much of 
the forest land has insufficient humus beneath the comparatively 
open stands of trees and there is within the forest a general absence 
of dense brush or of grass and of other herbaceous cover. This con- 
dition is far more prevalent in the mountains of the southern portion 
of the State than to the northward and *at lower altitude than at 
higher. The rainfall even in the mountains toward the south is in- 
sufficient to support forests, the cover of the San Bernardino and 
other mountains in this section being largely open stands of brush. 
(PI. 7.) Corrasion is active on the Tuolumne, the Stanislaus, and 
other streams of the Sierras which debouch into the Sacramento or 
the San Joaquin Rivers. This washing of stream banks is more or 
less constant, being a characteristic of streams coursing through un- 
consolidated soils or those deficient in cohesion, as are many of those 
of the California Basin. On the smallest tributaries this action 
might be lessened by protective planting. 
Violent irregularities in the character of the precipitation, espe- 
cially in the southern Sierras, add to the degrading influence of the 
streams, and make the lessening of erosion an important considera- 
tion. Although the general rainfall of California is light, 12 to 35 
inches, one-half of it normally falls during three months — Decem- 
ber, January, and February. In years of maximum precipitation 
the rainfall during this three-months' period will often equal or 
exceed that which would fall in the South Atlantic States during 
the same time. The rainfall of lower altitudes is frequently con- 
centrated, and this is especially favorable for erosion. Only at the 
higher elevations is a large part of it snoAv, and its melting is often 
forced by a warm rain. The snow fields are found only at the 
highest altitudes,and even on Mount Shasta and Mount Whitney, 
although they do much toward sustaining stream flow during the 
dry season, they are not perennial, and except in gulches and north 
slope canyons, largely disappear after midsummer. 
The functions of forest cover in limiting erosion are at an optimum 
at middle and lower altitudes in the Sierras of middle and southern 
California because of the character of the rainfall, which while suf- 
ficient to maintain a forest cover is irregular and frequently so con- 
centrated that destructive erosion takes place wherever the soil is 
not adequately protected by a blanket of humus. South of the higher 
mountains and in the foothills, the lower rainfall and high evapora- 
tion factor preclude forest growth, or at best permit only the growth 
of thin, open stands of woodland with humus, so scant that it is 
rapidly dissipated through nitrification and there is a general ab- 
sence of dense herbaceous and shrubby ground cover. In case the 
humus is destroyed grass does not form a sod. On the northern 
coast of California the conditions partake of those on the Columbia 
