Mr. Willis. 
390 DISCUSSION: FORESTS, RESERVOIRS, AND STREAM FLOW 
proportion of run-off, and it must be recognized that, in the semi- 
arid plain, narrow limitations are placed upon the advantageous growth 
of trees, which do not hold for the mountains under humid conditions. 
There, more water is stored by the slow-spilling surface and retained 
by the ground under the forest than by that under the quick-spilling 
slope of bare ground. The stored water in the ground is the principal 
source of low-water supply, and thus, in times of precipitation, the 
forest performs a gervice which inures to the advantage of low water 
in times of drought. 
The ideal condition for ground-water storage, in order to main- 
tain stream flow, is an even plain covered by a receptive soil, that is, 
by a soil which is in the best state of preparation for growing crops. 
Such a soil is maintained by deep plowing, by culture during the 
growing season, by keeping the fallow ground broken up, and by crop 
rotation; that is, by methods which increase the productiveness of 
the fields. The one essential condition, however, to make this surface 
more retentive than a forest-covered surface, is that it shall be so 
nearly level that run-off will not be a notable factor. To establish 
this condition on gentle slopes, the farmer may resort to contour 
plowing and harrowing, but to establish it on steep slopes, it is neces- 
sary to build terraces. All the older nations of the world have had 
to build terraces in order to extend the cultivated areas far enough 
up the mountain slopes to supply, in some measure, the demand of 
growing populations for food. We are still a long way from that 
condition, but nevertheless it should be recognized that terraces have 
a place in systems for the conservation of waters for city supply, 
power, or navigation. In a region where the conditions are so dif- 
ficult and so critical as they are in the Southern Appalachians, farm- 
ing should be carried on only as far as terracing can be adopted 
economically. Beyond that area, wherever terracing would be so 
costly as to prohibit farming, the surface should be covered with<a 
vigorous forest. 
The writer cannot leave this subject without commenting on the 
oft-repeated statement that natural re-forestation restores the con- 
ditions of water control, which existed under virgin forests. No 
one will deny that natural re-forestation is a general result of the 
initial cutting of the forest. No one can reasonably affirm, however, 
that the second growth which springs up behind the American lum- 
berman possesses the character of the virgin forest. Its roots are 
weaker, its branches afford less protection, and its influence upon 
water storage is different. In some cases, the young forest being more 
open than the old, and its demand for water less great, a larger pro- 
portion of water will appear in the springs, and the rivers will run 
fuller for a time. This is particularly true of the densely forested 
ranges of the Northwest, and Colonel Chittenden cites a special case 
