66 FOREST LANDS FOR THE PROTECTION OF WATERSHEDS. 



the former low-water flow of a spring or rivulet and what it is now is rela- 

 tively an insignificant quantity. Most of such water sources yield but a small 

 fraction of a cubic foot per second. Whether these small quantities are a trifle 

 more or less cuts very little figure in the aggregate; and so it counts but little 

 In the flow of a great river whether some of its extreme sources lose a portion 

 of a volume that is already inappreciable. When the summer showers come, 

 however, there is a marked difference. At such times the forests not only hold 

 the water back they often swallow it completely. Small showers that make 

 a perceptible run-off in the open are often practically all absorbed in the leaves 

 of the trees. Heavier showers, that make freshets in the open, are largely 

 absorbed in the leaves and forest bed and pass off in evaporation ; so that, 

 contrary to the general view, the evaporation from the forest is greater at 

 such times than in the open country and the run-off from summer precipitation 

 is less. A single shower may produce a sufficiently greater run-off in a de- 

 forested area to more than offset the diminished low-water flow for several 

 weeks. Now, on most of the smaller streams quantity of flow is a more impor- 

 tant matter than natural uniformity of flow, particularly in the summer time. The 

 day of the small mill, which was so dependent upon such uniformity, is past. 

 The modern water power invariably seeks uniformity by artificial regulation, 

 and the ups and downs of its sources of supply are abolished in its storage. 

 Therefore it does not matter nearly as much that the run-off of the small 

 streams be uniform as that it yield a good flow of water ; and if forests dimin- 

 ish the total low-water supply, this fact more than offsets the gain in uniform- 

 ity. Likewise the great rivers swallow up and equalize the small irregularities 

 of their headwaters and actually experience a somewhat larger low-water flow 

 than if their watersheds were still thickly forested. Thus, while forests may 

 decrease somewhat the extreme range between maximum and minimum run-off 

 on very small watersheds, they do not do so on great ones, which are combi- 

 nations of very small ones. At the same time it seems certain that forests 

 decrease somewhat the total run-off from watersheds, small or great. 6 



Influence of forests upon snow melting. The second proposition that for- 

 ests have a beneficial effect upon the run-off from snow melting is quite as 

 firmly fixed in the popular belief as that just considered, but has even less 

 foundation in fact. It is a relation that can be definitely traced, and it can 

 be demonstrated that the effect of forests upon the run-off from snow is invari- 

 ably to increase its intensity. This results from two causes, one affecting the 

 falling of the snow and the other its melting. 



In the first place, forests break the wind, prevent the formation of drifts, 

 and distribute the snow in an even blanket over the ground. In the open coun- 

 try, the snow is largely heaped into drifts, their size depending upon the con- 

 figuration of the ground, the presence of wind-breaks, and the prevalence and 

 force of the wind. These drifts form admirable reservoirs and in the high 

 mountains are the most perfect known. Forests prevent their formation 

 entirely. 



The period of snow melting begins in the open country much earlier than in 

 the forests. At first the melting is due mainly to the direct action of the sun's 

 rays before there is sufficient warmth in the general atmosphere to produce any 

 effect. The thinly covered areas melt off first and the streams experience a 

 diurnal rise and fall following the warmth of day and the frost of night. 

 Nothing like a flood ever arises from such melting. 



Under forest cover this action is interfered with more or less, depending upon 

 the density of the shade. Even after the ground in the open is entirely bare, 

 except under the drifts, the forest areas may still be covered with an unbroken 

 layer of snow. It is generally, though erroneously, considered that this delay 

 Is beneficial, by carrying farther into the summer the release of the winter pre- 



So far as the author is aware, Col. T. P. Roberts, of Pittsburg, Pa., was 

 the first to call attention to this characteristic of stream flow. 



6 This subject was ably discussed by Mr. llaphael Zon, of the Forest Service, 

 Department of Agriculture, in Transactions, Am. Soc. C. E. f Vol. LIX, pp. 

 494-495. He states, among other things, that " the quantity of water available 

 for stream flow from forested watersheds, all other conditions being equal, 

 Is less than from nonforested watersheds;" that "the forest soil receives 

 least precipitation, next conies meadow land, and lastly tilled land:" 

 that " in the forest, only the upper layer of the soil is moister than in the open, 

 the lower layers being always drier." This discussion is well worth perusal. 



