FOREST INFLUENCES. 315 



4 



The forest, then, even if under unfavorable topographical and soil conditions (steep slopes and 

 impermeable soils) it may not permit larger quantities of water to drain off underground and in 

 springs, can yet influence their constancy and equable flow by preventing loss from evaporation. 

 (Pp. 137-140, Bui. 7.) 



(3) The surface drainage is retarded by the uneven forest floor more than by any other kind 

 of soil cover. Small precipitations are apt to be prevented from running off superficially through 

 absorption by the forest floor. In case of heavy rainfalls this mechanical retardation hi connection 

 with greater subterranean drainage may reduce the danger from freshets by preventing the rapid 

 collection into runs. Yet in regions with steep declivities and impermeable soil such rains may 

 be shed superficially and produce freshets in spite of the forest floor, and an effect upon water 

 conditions can exist only from the following consideration. (Pp. 140-159, Bui. 7.) 



(4) The well-kept forest floor, better than even the close sod of a meadow, prevents erosion 

 and abrasion of the soil and the washing of soil and detritus into brooks and rivers. 



This erosion is especially detrimental to agricultural interests as well as water flow in regions 

 with this surface and impenetrable subsoils, and where rains are apt to be explosive in their 

 occurrence, as in our western and southern country. The best soil of the farms is often washed 

 into the rivers, and the water stages of the latter by the accumulations of this soil are influenced 

 unfavorably. (Pp. 159-162, Bui. 7.) 



(5) Water stages in rivers and streams which move outside the mountain valleys are dependent 

 upon such a complication of climatic, topographic, geological, and geographical conditions at the 

 head waters of their affluents that they withdraw themselves from a direct correlation to surface 

 conditions alone. Yet it stands to reason that the conditions at the head waters of each affluent 

 must ultimately be reflected in the flow of the main river. The temporary retention of large 

 amounts of water and eventual change into subterranean drainage which the well-kept forest 

 floor produces, the consequent lengthening in the time of flow, and especially the prevention of 

 accumulation and carrying of soil and detritus which are deposited in the river and change its 

 bed, would at least tend to alleviate the dangers from abnormal floods and reduce the number and 

 height of regular floods. (Pp. 162-170, Bui. 7.) 



Note. — Concerning the moisture of the soil the results of the most recent experiments differ. Rainann, in 1895, 

 published a series of results which indicated that the soil of the forest may be even drier than that of the neighbor- 

 ing open land. This view he finds strengthened by experiments made in small clearings within the forest, where 

 ho finds the soil of the sunny bide of the clearing and that of the old forest itself decidedly drier than the soil of the 

 shaded part of the clearing, though he also finds the soil under a young bush cover more moist than that under old 

 timber. 



Whether a forest cover aids m the accumulation of ground water by improving the j)ermeability of the soil 

 was made the object of au expeiiment by Wolluy in a series of inconclusive small pot experiments which led this 

 investigator to the questionable result that bare land was more conductive to percolation than ground covered 

 either by grabs or tiees. This would surely be true only if the bare ground, as in the experiments, is kept in an 

 artificial, not natural condition. 



Attempts to deduce the influence of forest on water flow from wholesale measurements and observations have 

 been made in this country by Vermeulo, of New Jersey (see Proceedings American Forestry Association, Vol, XI, pi 

 130-137, and report of N. J. Geological Survey, 1894), and Rafter, of New York (Proceedings of American Forestry 

 Association, Vol. 12, pp. 139-165, and report of State engineer and surveyor of New York, 1896), the former claiming 

 that no appreciable influence existed, the latter calculating the influence of the forest to be equal in value to o or 6 

 inches of rainfall, this amount of moistuie being saved by its presence. 



Among recent papers which possess the highest value in describing the movements of water in the ground, 

 and thus throw light on a most important phase of the whole subject, Bulletin 32 of the Experiment Station, Fort 

 Collins, Colo., by Piof. L. G. Carpenter, is noteworthy. Professor Carpenter shows that it is possible by mechanical 

 means (ditches in this case) to prevent the rapid run off in high- water time and thus produce a steadier flow of a 

 stream and also raise the level of the ground water, as well as saturate large areas of otherwise arid land. In other 

 words, he shows that in Colorado the work of irrigation has resulted in a rise in the level of the ground water, 

 changing deep wells into shallow ones; that it has taken water out of the Platte and Cache la Poudre rivers and 

 saturated thousands of acres of formerly arid land, the seepage of which has changed dry branches into steady 

 rivulets and supplies already a steady inflow into the rivers, from which the water is taken above the fields. This 

 inflow tends to make these rivers steady and uniform sources of water supply and makes irrigation possible at points 

 below where in former times such irrigation would have been out of the question. 



