1905 



FORESTRY AND IRRIGATION 



167 



the denuded basin the discharge dur- 

 ing thq same period was practically 

 nothing. 



This is by far the most far reach- 

 ing and generally accepted influence 

 of forests upon stream supply. Its 

 extent and limits under varying con- 

 ditions are still unknown. Its ability 

 to counteract the effect of other un- 

 favorable factors, such as steep to- 

 pography and unevenly distributed 

 rainfall, is still largely conjecture. We 

 can, however, state with certainty 

 that forests exert a powerful though 

 ill defined influence in this direction. 



A third distinct influence has been 

 attributed to forest cover upon a 

 drainage basin, namely, the checking 

 of erosion upon the watershed and the 

 consequent silting of stream channels, 

 where the silting of storage reservoirs 

 Forest cover is supposed to check 

 erosion by lessening the amount and 

 velocity of surface run-off. Further- 

 more its mat of roots, humus, litter, 

 moss, and undergrowth tends to hold 

 the soil in place and protect it from 

 the wearing action of such water as 

 is discharged over or near the surface. 

 Evidences of this influence are at hand 

 in any mountain region of the United 

 States. It is especially marked in the 

 Southern Appalachians in the turbid- 

 ity of streams which drain denuded 

 "basins, the choking of their channels 

 with silt, and the often destructive 

 deposits of mud and debris over level 

 bottoms along their lower courses. 

 This factor has a direct bearing upon 

 the reclamation project in the west 

 valley bottoms, and storage reservoirs, 

 is often a very serious obstacle to the 

 control of streams. 



Not only does direct erosion of the 

 exposed surface follow the removal 

 of forest cover, but this loss of soil 

 means ultimately the loss of the por- 

 ous surface layer in which rainfall is 

 absorbed and converted into under- 

 ground seepage. Loss of forest cover 

 and loss of soil go together. As the 

 process of erosion goes on, imperma- 

 able strata are exposed, resulting in 

 the rapid and destructive discharge of 



precipitation as surface run-off. For- 

 est cover on a drainage basin, there- 

 fore, not only checks directly the silt- 

 ing process so destructive to stream 

 beds, valley farms, and reserviors, but 

 also by conserving the porous soil 

 layer affects directly the regularity of 

 stream flow. 



Now it must be constantly borne in 

 mind that these influences of forest 

 cover, in their present status, must be 

 classer as "attributed" rather than 

 demonstrated or proved. Available 

 material bearing upon them consists 

 largely, up to the present, of theoreti- 

 cal discussion. Such experimental 

 data as has been collected is fragmen- 

 tary and inconclusive. Among in- 

 vestigators there are radical differ- 

 ences of opinion. At the same time 

 we may consider the general equaliz- 

 ing effect of forest cover upon stream 

 flow as well established in principle, 

 though vague and ill defined in de- 

 tail application. 



Another point cannot be too strong- 

 ly emphasized, that forest cover is 

 but one of man}' factors in a drainage 

 basin whose combined influence pro- 

 duces a stream of a given character. 

 Great care is required not to attribute 

 to forests influences which are .due 

 primarily to other factors ; and like- 

 wise not to assume that forests exert 

 no such influence when it simply may 

 be counteracted by other and more 

 powerful factors. 



It is moreover impossible to make 

 generalizations which can be applied 

 to a wide range of natural conditions. 

 This problem should rather be attack- 

 ed piecemeal, by careful, detailed 

 studies of individual streams and 

 limited areas. Such studies should 

 cover all the features of the drainage 

 basin which may affect stream flow, 

 precipitation, topography, geological 

 conditions as well as forest cover, to- 

 gether with the behavior of the stream 

 as shown by continuous discharge 

 measurements. The various factors 

 which govern stream flow are so close- 

 ly interrelated in their action that it 

 is impossible to assign to each its ex- 



