278 FORESTS, RESERVOIRS, AND STREAM FLOW 
It often escapes attention, except with those who are in the woods 
a great deal, that the water establishes little channels through the 
débris where the latter is of long accumulation and somewhat perma- 
nent in character. Such débris does not in reality offer so great an 
obstruction to flow as one would suppose, and as would be the case if 
its condition underwent frequent change. 
The statement is constantly met that forests are very efficacious in 
the protection of river banks from undermining and steep slopes from 
sliding. The exact reverse is the case. As every river engineer knows, 
nothing is more disastrous to a river bank on an alluvial stream than 
heavy trees. This is due partly to their great weight, but in large part 
to the swaying effect of the wind and the enormous leverage of the 
long trunks which pry up the ground and facilitate the tendency to 
undermining. One of the regular policies of river control is to cut 
down these trees for a distance back from the edge of the bank wherever 
complications with private ownership do not prevent. Snags and drift- 
wood in the channels have always been among the most serious ob- 
stacles to navigation on streams flowing between forest-covered banks. 
Likewise where railroad or highway grading cuts the skin of unstable 
mountain slopes, the presence of large trees immediately above tends 
powerfully to loosen the ground and cause it to slide; and in such cases 
it is necessary to cut down the timber. Far better than forest trees 
on river banks are thick growths of willow, alder, or any of the smaller 
close-growing shrubs; and on side hill slopes either such shrubbery 
or a good turf.* 
In the current discussion a great deal is made of the fact that 
mountain slopes are “quick spilling,” the deduction being that they 
therefore are more productive of floods. This is quite contrary to the 
fact. It is perfectly true that more rain falls on the hills than on the 
lowlands, that a greater percentage of rainfall runs off from steep 
than from flat slopes, and that it runs off more rapidly; but it does 
not follow at all that these conditions produce greater floods. A moun- 
tain stream carries off the water within its banks a great deal faster 
*The following testimony before the Board of Consulting Enginee 
ee is he ine pent (Report, p. 329): z = ‘ Hy ROBINS 
uestion by Mr. Welcker: “Mr. Chairman, I would like t 
thinks that vegetation prevents the sliding?” SBE 1 Mike DEneny 
Mr. Dauchy: “My experience has been the reverse; I have stopped sliding hills 
by cutting off the vegetation. The weight of the timber on 
materially to assist the sliding.” Selling ‘slope: aide 
Mr. Welcker: ‘‘Does not the vegetation diminish it?” 
Mr. Dauchy: “If you could get a grass-covered sl 2 
minish it.” e slope it would help to di 
