FOREST COVER IN PROTECTING RESERVOIRS 33 
velopments, reservoirs are being located on their headwaters for 
the purpose of storing storm waters and thus equalizing the flow of 
the streams and obtaining a more thorough utilization of power. 
If it were ascertained that the pondage of a reservoir located on 
a head stream was in danger of being materially reduced through 
erosion of soil from denuded land, it would be justifiable from an 
investment point of view to expend in the preservation of the storage 
capacity or in the extension of its life a sum of money which would 
not exceed the present value of the prolonged life of this storage 
capacity. 
The acquisition of the denuded land on which erosion is most 
excessive and the planting of it to trees, not only offer means of 
greatly reducing the erosion, but promise returns from the invest- 
ment so made. If such denuded lands can be acquired at a sufficiently 
Ioav price, the returns from the growth of the timber may be ex- 
pected to pay for the entire cost, with interest, of the land and its 
afforestation. Under some conditions the cost of the land and its 
afforestation would be so high that the growth of the timber would 
probably not return a current rate of interest on the investment. 
In such cases it would be economically desirable for the additional 
cost of the land and afforestation to be charged against the pro- 
longed utility value of the reservoir. There would then be justifica- 
tion for expending in this way such a sum as would equal the present 
value of the prolonged life of the reservoir. 
One of the most important considerations in determining the 
location of reservoirs for such a purpose is the life of the reservoir 
and the rate at which the initial pondage is lost through siltage due 
to erosion of soil. 
The streams in the different parts of the United States may be 
grouped in relation to the quantity and irregularity of their run- 
off, their turbidity, and the value of forest cover in protecting their 
headwaters, and thus influencing the quantity of their solid burden. 
In such a classification it appears that, because of the character 
of the soil, the rather small amount and even distribution of the 
precipitation, and the extensive storage of water in lakes and moun- 
tain snow fields, forests exercise a generally low influence in the 
Northeast, in the Lake regions and in the Northwest. 
At the other extreme are the rivers of the Great Plains and Texas 
and the Colorado Kiver. These streams flow through regions which, 
on account of the extreme irregularity and scantiness of the rainfall, 
are largely treeless or wooded only with scattered groves, and in 
consequence the influence of the forest cover is negligible. 
There is an intermediate type of stream on whose watershed the 
forests do exercise a most important function in the protection of 
reservoirs. This type embraces the streams of the southern Appa- 
lachians, those of the southern Piedmont region, most of those that 
rise in the mountains of southern California, and the headwaters 
such of those as have their sources in the southern part of the 
Rocky Mountain system. They are within regions of irregular and 
usually heavy rainfall. Their headwaters at least are in forest 
regions. Wherever extensive areas have been denuded and exposed 
