FOREST COVER IX PROTECTING RESERVOIRS 29 
giving fullest opportunity for entire absorption with a very small 
amount of surface run-off. The winter snowfall is heavy, especially 
in the eastern mountains; and the deep snow banks in gulches and 
north hollows and where protected by the dense forest of conifers 
from sun and wind melt gradually during spring and summer, main- 
taining a steady stream flow. This not only compensates for the 
deficiency of the summer rains, but the absence of such rains is 
favorable, since they would accelerate the melting of the snow. The 
snow drifts and fields of the high peaks of the Cabinet, Coeur 
d'Alene, and Bitter Eoot Mountains, and of the Continental Divide 
sometimes last until reinforced by those of the succeeding winter. 
In the Williamette the snow-water flow is uninterrupted, for it is 
largely fed by the glaciers and perpetual snow fields of Mount 
Hood and Mount Jefferson, just as Lewis River is fed from the snow 
fields of Mount Adams. (PL 18.) 
The torrential spring flood of the Northeastern States from the 
warm spring rains on the winter's snow, and the high, early summer 
flow of the streams of the Southwest from the rapid melting of the 
mountain snow under the high temperature of early summer, are both 
much reduced on the Columbia. The freshet season extends from 
May to July. The melting of snow in the mountains of Idaho and 
Montana is usually a gradual process, frequently occupying the entire 
summer. Rarely there may be accumulated snow in the forests and 
a late spring followed by very warm rains producing high floods such 
as those of 1826, 1854, and 1894. The northern tributaries are less 
erratic in their flow than the southern, and several lakes add to their 
stability of flow. Occasionally, however, a warm chinook wind will 
melt much of the snow during April, or even in March, causing high 
turbidity and floods on streams like the Clearwater and Lewis Fork. 
These winds, however, are exceptional and usually of short duration. 
The flood season, when the streams are very muddy, is usually two 
months later. The extension of cultivation has had little effect in 
increasing turbidity, since the cultivated lands are largely in the 
more level valleys and on account of the character of the precipi- 
tation. 
Although the equable flow of these streams is primarily a result of 
the climatic conditions, the influence of the mountain forests is 
serviceable in protecting the snow from evaporation and lessening the 
rapidity of its melting. It is of greater value in checking erosion, 
notwithstanding the prevailing porous soils of the mountains, while 
the large amount of brush beneath the timber and the readiness with 
which grass, other herbage, and shrubs come in on naked soil are 
important factors. 
_ On the Plains the rainfall is of such character, both in distribu- 
tion and amount, that there is scant erosion. Although the silts of 
the Priest, the Pend Oreille, and other agricultural river valleys of 
the eastern headwaters of the Columbia are incoherent, the limited 
rainfall precludes the possibility of excessive erosion, unless at times 
of concentrated precipitation, a character of rainfall to which this 
region is seldom subjected. Should erosion of these soils take place, 
however, the placid portion of the Pend Oreille and other streams 
offer suitable situations for silting. 
