DISCUSSION : FORESTS, RESERVOIRS, AND STREAM FLOW 393 
and also throughout Europe and not a small part of China. There Mr. Willis. 
is, according to his observation, a definite relation between the different 
valley deposits and forest-covered hills on the one hand or denuded 
hills on the other. He cannot recall an exception to the fact that 
streams flowing from hills which have been denuded of forests have 
filled their lower valleys with broad flats of sand and gravel through 
which the waters meander in shifting, uncertain channels. Equally 
closely related are forested mountains and clear streams, which flow 
through valleys with well-established channels and more or less ex- 
tensively developed flood planes that are covered with appropriate 
vegetation. 
In all regions where precipitation is sufficient to maintain a forest 
cover, a certain equilibrium is reached between the rate of soil pro- 
duction and soil erosion. Streams of a certain size, on a certain 
slope, are able to keep their bottoms clean and slowly to erode 
them. Less efficient rills are unable to penetrate the protective 
cover and cannot effectively begin the task of corrasion. This limit 
varies according to climate, vegetation, and soil conditions, and is 
not the same throughout any one district, much less in regions so 
different as the Appalachians and the Cascade Mountains; but wher- 
ever virgin forests have persisted during a geologic age, as they have 
over a great part of the United States, the processes of Nature have 
brought about the adjustment, and have reduced the progress of erosion 
to a slow rate. 
This adjustment is gravely disturbed when Man cuts a forest. No 
material damage need ensue if he does so intelligently and protects 
the surface until vegetation can recover from the relatively slight 
injury that is done by forest cutting under good methods; but the 
methods of the American lumberman, followed by fire, as they too 
often are, result in such a disturbance of the adjustment as to make 
recovery extremely difficult, even if possible. 
The effect of the disturbance is seen in the vigorous growth of 
thousands of little gullies, which are scarcely noticeable at first, but 
which grow like the branches of a tree, and spread far out into the 
previously uneroded surface. Vegetation always makes a brave fight, 
but unless aided by Man it can never recover the position which it 
originally held. 
As stated by Colonel Chittenden, Man too often does not aid vege- 
tation, but carelessly establishes fresh channels of erosion in wheel 
ruts, logging chutes, etc.; he cultivates the ground under conditions 
that promote the gathering of the rills and the washing of the soil. 
Thus, he proceeds with criminal negligence to accelerate the process 
which, if continued, will eventually make such districts as the Ap- 
palachian Mountains uninhabitable. 
Any one who takes a broad view of this vital question of the 
