FOREST LANDS FOR THE PROTECTION OF WATERSHEDS. 65 



Now, in nature this ideal illustration is never fully exemplified in the cleared 

 land and the forest. There is nearly everywhere a marked retentive capacity in 

 the bare soil. In newly plowed ground it is probably greater than in the forest. 

 Moreover, certain crops, like heavy grass or grain, obstruct the flow of water 

 almost as much as the forest cover. On the other hand, the furrows of culti- 

 vated fields, drainage ditches, roads, and particularly the pavements and roofs 

 of towns, greatly accelerate the run-off; so that, while the full contrast of the 

 ideal example does not exist in nature, the principle of the illustration applies 

 perfectly. That is, there are times when the percentage of retention in the 

 forest bed is 0, and there are other times when it is 100; or, there are times 

 when so much water comes that the forest bed can hold none of it, and there 

 are times when so little comes that it holds it all. Between these extremes 

 there are periods when it holds more or less and gives up less or more and exer- 

 cises a corresponding influence upon the run-off. There is another important 

 condition not exemplified in the illustration, and that is that the forest areas 

 are scattered everywhere, the ground has an infinite variety of slope, the show- 

 ers never fall uniformly over an entire watershed, and the final result in the 

 total run-off is the summation of thousands of tributary results. 



It is true, therefore, as popularly understood, that in periods of ordinary 

 rainfall, with sufficient intervals for the forest bed to dry out somewhat, forests 

 do exert a regulative effect upon run-off. They modify freshets and torrents 

 and prolong the run-off after storms have passed, and thus realize in greater or 

 less perfection the commonly accepted theory. 



This result utterly fails, however, in those periods of long-continued, wide- 

 spread, and heavy precipitation, which alone cause great floods in the large 

 rivers. At such times the forest bed becomes completely saturated, its storage 

 capacity exhausted, and it has no more power to restrain floods than the open 

 country itself. Moreover, the fact that the forest bed has retained a portion of 

 earlier rainfall and is yielding it up later to the streams, produces a condition 

 that may be worse than it would be in a country cleared of forests. Really 

 great floods in large rivers are always, as is well known, the result of combina- 

 tions from the various tributaries. It is when the floods from these tributaries 

 arrive simultaneously at a common point that calamitous results follow. Any 

 cause which facilitates such combinations is, therefore, a source of danger. 

 Now, unquestionably, in a heavily wooded watershed forests do have a tendency 

 in this direction. When a period of heavy storms occurs, spreading over a great 

 area, continually increasing in intensity, the forests, by retaining some portion 

 of the earlier showers and paying them out afterwards, do produce a general 

 high condition of the river which may greatly aggravate a sudden flood arising 

 later from some portion of the watershed. That the forest does promote tribu- 

 tary combinations there would seem to be no question, and that it may therefore 

 aggravate flood conditions necessarily follows. It is not contended that this 

 increase is ever very great, but it is contended that forests never diminish great 

 floods and that they probably do increase them somewhat. The forests are 

 virtually automatic reservoirs, not subject to intelligent control, and act just as 

 the system of reservoirs once proposed by the French Government for the con- 

 trol of floods in the River Rhone would have acted, if built. These reservoirs 

 were to have open outlets, not capable of being closed, which were intended to 

 restrain only a portion of the flow. A careful study of their operation in cer- 

 tain recorded floods showed that they would actwally have produced combina- 

 tions more dangerous than would have occurred without them. 



Consider now periods of extreme drought and grant that as a general rule, 

 springs and little streams dry up more completely than when forests covered 

 the country, although this difference is very greatly exaggerated in the pop- 

 ular mind. At first thought one would conclude that, since the springs and 

 streams make up the rivers, these also ought now to show a smaller low-water 

 flow than formerly. This, however, is not the case. The difference between 



a The term "as a general rule," is used, for it is by no means absolute. In 

 particular the drainage of low swamp lands leads off into the streams, in dry 

 weather, waters that formerly remained or passed off in evaporation, and in 

 such cases even the low-water How is greater than it used to be. In lSl)f> the 

 author saw an example of this on the Scioto River near the outlet of the great 

 Scioto swamp, which had recently been drained. A small mill was able to 

 operate during the low-water season more regularly than formerly. Tile drain- 

 age, now so widely used, has the same tendency. 



7253S AGE 09 5 



