48 FOREST LANDS FOR THE PROTECTION OF WATERSHEDS. 



the forests from other influences which affect the flow. Let me illus- 

 trate by a very homely and unpoetic illustration. If I wish to find the 

 effect of tea and coffee on my child, who has never taken tea or coffee, 

 I do not give tea and coffee together. If I give her tea and coffee 

 together and there is an effect, I am utterly in the dark ; it may be due 

 to the coffee and not at all to the tea ; it may be due entirely to the 

 tea and not to the coffee. It may be due partly to the tea and partly 

 to the coffee. It may be due to neither separately, but simply to the 

 fact that the two have come together. The influences which affect 

 the flow of streams and floods are varied, and the other influences are 

 more important than forests, for instance, rainfall. There is never 

 a great flood without a great rainfall. The distribution of that rain- 

 fall during the year is another very important element. 



Take the case, for instance, in countries where the rainfall is prin- 

 cipally in the form of snow, in the winter and spring. It goes off and 

 forms a flood in the spring. There is little rainfall during the sum- 

 mer, and the springs get very dry, and the streams practically are 

 dry all during the summer. Perhaps the very next year there may be 

 the same amount of rainfall in the year, but it may be distributed 

 differently. There may be very little in the spring and winter, but 

 there may be a large rainfall in the summer. I was looking at a re- 

 port of a rainfall the other day in which there had been in two con- 

 secutive years the same rainfall, and yet in one year there was a run- 

 off of 12 inches and in the next year, the same rainfall, and run-off 

 with 17 inches. As I say, the influences which are due to the distri- 

 bution of the rainfall, and so forth, are more important than the 

 forests, but the forests constitute an influence which can be controlled. 

 There are just two elements which enter into the problem which can 

 be controlled. The rainfall can not be controlled; the distribution 

 of the rainfall through the year can not be controlled. All these 

 meteorological phenomena, varied as they are, of course are entirely 

 beyond the control of man. The forests can be controlled, and the 

 other element which could be controlled is the storage. By forests 

 and storage together the flow can be regulated to the greatest possible 

 degree. The storage alone, without ,the control of the forests, would 

 itself be rather futile, because if the trees are cut down erosion follows 

 and the reservoirs are more apt to fill up with silt, and in time to lose 

 their power of storage because they lose their capacity. 



President Van Hise has referred to the important study which has 

 been made the past summer in regard to the rainfall. It is a very 

 remarkable thing that during the last ten years of the period studied 

 there were more days of flood than during the first twelve years, and 

 perhaps a cursory examination would make one come to the conclu- 

 sion that there had been less floods, therefore, and that cutting down 

 the trees in the valley had been a fatal influence on the floods. But 

 when you study the rainfall you will see the explanation, and the 

 thing that Mr. Leighton has done has been to combine those two as 

 they have never been combined before. He has taken the number of 

 rain storms which are sufficient to produce a flood, and he has com- 

 pared those with the number of days of flood, and the result is per- 

 lectly definite, as Doctor Van Hise has stated. It shows an increase 

 of flood in proportion to the days of rainfall, or in proportion to the 

 number of rain storms, of about 18.75 per cent. 



