Mr. Swain. 
366 DISCUSSION: FORESTS, RESERVOIRS, AND STREAM FLOW 
and at another time deforested. It must also be borne in mind that 
other factors in the problem are more important than the forests, for 
instance, the amount and, particularly, the distribution of the rainfall. 
When, in two successive years, with practically the same rainfall in 
each year, the same water-shed shows a run-off of about 12 in. in one 
year and about 17 in. in the next, the difficulty of the problem will 
be apparent. 
However, means of forming reliable opinions upon the more im- 
portant aspects of this matter are not wanting, although some minor 
points will probably long remain matters of opinion. 
The author first considers the relation of forests to stream flow, and 
states that: 
“The commonly accepted opinion is that forests have a beneficial 
influence on stream flow: 
“(1) By storing the waters from rains and melting snow * * * 
preventing their rapid rush to the streams and paying them out 
gradually afterward, thus acting as true reservoirs in equalizing the 
run-off, 
“(2) By retarding the snow-melting in the spring and prolonging 
the run-off from that source. 
“(3) By increasing the precipitation. 
“(4) By preventing erosion of the soil on steep slopes and thereby 
protecting watercourses, canals, reservoirs and similar works from 
accuroulations of silt.” 
This will probably be admitted to be a fair statement of what the 
believers in the benefits of forests consider to be true, except that 
some do not think that there is yet sufficient demonstration that they 
increase the rainfall, although the writer believes that they do. 
With reference to the first of these points, the author states that it 
is “strictly true for average conditions.” He says: “It is true, there- 
fore, as popularly understood, that, in periods of ordinary rainfall, with 
sufficient intervals for the forest bed to dry out somewhat, forests do 
exert a regulative effect upon run-off. They modify freshets and tor- 
rents and prolong the run-off after storms have passed, and thus 
realize in greater or less perfection the commonly accepted theory.” 
He believes, however, that this beneficial effect is not exerted under 
extreme conditions, 7. e., great floods and excessive low waters. 
After the above admission, it would seem scarcely necessary to 
discuss this portion of the subject further, unless, as the author seems 
to believe, regulation under all but extreme conditions is of little 
benefit. Aside from the fact that it is a benefit to have the flow regu- 
lated, even to a small extent, during the greater part of the time, the 
writer cannot agree with the author that extreme conditions alone 
determine the character and cost of river control. The erosion of the 
land, the washing away of banks, the silting up and changing of 
channels, and the destruction of property, occur in the case of mod- 
erate floods as well as in the case of excessive floods; although, of 
