64 FOREST LANDS FOR THE PROTECTION OF WATERSHEDS. 



that forests in the eastern portion of the United States have disappeared to a 

 large extent within the past century. The assumption is that floods and low 

 wafers in the same region are more frequent and severe than before the forests 

 were cleared away. The conclusion is that these assumed conditions must be 

 due to the disappearance of the forests. Post hoc, ergo propter hoc is the argu- 

 mentative process relied upon, and little effort is made to consider whether 

 there may not be some other and more satisfactory explanation. The author 

 will attempt to analyze the problem from a theoretical standpoint, and will then 

 cite existing records so far as these are sufficiently long continued to be worth 

 anything. He will consider, first, the effect of the forests where stream flow 

 results from rain alone, and, next, where it results in part from melting snow. 



Effect of forests upon the run-off from rainfall. The first of the above proposi- 

 tions the retentive action of the forest bed may be accepted at once as strictly 

 triu 1 for average conditions. It is not true for extreme conditions great floods 

 and excessive low waters the conditions that determine the character and cost 

 of river control. Consider an inclined-plane surface, practically impervious to 

 water, with a layer of sand covering some small portion of it, and let a uniform 

 spray of water be applied to the entire surface. Assume that the temperature 

 and rate of evaporation are relatively low. As soon as the spray begins water 

 commences to flow from the uncovered surface, but not for a time from that 

 covered by the sand. After a while it begins to trickle from the sand, increasing 

 in volume until the sand is thoroughly saturated, after which it flows off in as 

 great quantity per unit area as from the uncovered portion. If the spray is 

 stopped the water immediately ceases to flow from the uncovered area, but 

 continues in diminishing quantity from the covered area until it finally ceases 

 altogether ; but not all the water that fell on this area has run away. The sand 

 has retained some portion of it and given it off in evaporation, so that the total 

 run-off per unit area is somewhat less than on the uncovered portion. If the 

 shower be long-continued and the rate of evaporation very low, the difference of 

 total run-off per unit area from the two surfaces will be very slight. 



Suppose now that the temperature and rate of evaporation are high and that 

 the spray works intermittently. If the showers are small in volume and the 

 intervals between them long, the sand may retain nearly or quite all of the 

 individual showers and give them off in evaporation, so that there \\ill be no 

 run-off whatever. 



Between these two extreme conditions the covered area will exert a greater 

 or smaller regulative effect upon the run-off. The retentive power of the sand 

 will be less as the slope of the surface upon which it rests increases, or it will 

 be greatest when the surface is nearly horizontal and least when it is nearly 

 vertical. 



Since the above was written the author has noticed, in the report of the 

 hearing on House resolution 208 before the Committee on the Judiciary, that 

 Gifford Pinchot, Associate American Society of Civil Engineers, Chief of the 

 Forest Service, used an illustration very similar to that given above, except 

 that he failed to carry it to its logical conclusion. Addressing the committee 

 February 27, 1908, he said : " I have in my hand here a photograph of a denuded 

 hillside. After the forest has been removed rain falls on that hillside and runs 

 off rapidly, as the water I drop upon the photograph does now, and disappears 

 instantly. [Illustrating.) If, on the other hand, I place a forest cover on the 

 hillside that is exactly analogous in texture and effect with this piece of blot- 

 ting paper and drop the water slowly upon it, we would find that, instead of 

 running off slowly at the bottom, the water is held. [Illustrating with blotting 

 paper.] Part of it runs off, but as soon as the. absorbent quality of the paper or 

 the forest floor has time to take effect the water is kept and drips gradually for 

 a considerable length of time off the hill into the stream. This is an exact illus- 

 tration of the way in which the forest controls the stream flow on that hill- 

 side." 



Mr. Pinchot should have completed his illustration. He should have contin- 

 ued to sprinkle the paper long enough and heavily enough to have saturated the 

 paper completely, in order to show that the water would then flow from the 

 paper as rapidly as from the uncovered area; and he should then have ex- 

 plained that this condition represents what always happens in the forest in 

 times of great flood. Then he should have sprinkled the paper intermittently in 

 small quantities, and at such long intervals that the warm air of the room 

 would evaporate all of the absorbed water, and that none whatever would flow 

 away. He should then have explained that this condition represents what 

 always takes place in the forest in tirues of great drought. 



