84 FOBEST LANDS FOB THE PEOTECTION OF WATEBSHEDS. 



at such times exert no restraining effect upon floods, but, by virtue of its un- 

 controlled reservoir action, may actually intensify them. 



(3) In periods of extreme summer heat forests operate to dimmish the 

 run-off, because they absorb almost completely and give off in evaporation 

 ordinary showers which, in the open country, produce a considerable temporary 

 increase in the streams; and therefore, while small springs and rivulets may 

 dry up more than formerly, this is not true of the larger rivers. 



(4) The effect of forests upon the run-off resulting from snow melting is to 

 concentrate it into brief periods and thereby increase the severity of freshets. 

 This results (a) from the prevention of the formation of drifts, and (&) from 

 the prevention of snow melting by sun action in the spring, and the retention 

 of the snow blanket until the arrival of hot weather. 



(5) Soil erosion does not result from forest cutting in itself, but from cultiva- 

 tion, using that term in a broad sense. The question of preventing such 

 erosion or soil wash is altogether one of dispensing with cultivation or properly 

 controlling it The natural growth which always follows the destruction of a 

 forest is fully as effective in preventing erosion, and even in retaining run-off, 

 as the natural forest. 



(6) As a general proposition climate, and particularly precipitation, have 

 not been appreciably modified by the progress of settlement and the consequent 

 clearing of land, and there is no sufficient reason, theoretically, why such a 

 result should ensue. 



(7) The percentage of annual run-off to rainfall has been slightly increased 

 by deforestation and cultivation. 



If the foregoing propositions are correct they enforce two very important 

 conclusions one relating to the regulation of our rivers and the other to 

 forestry. 



It follows that no aid is to be expected in the control or utilization of our 

 rivers, either for flood prevention, navigation, or water power, by any prac- 

 ticable application of forestry. Remember always that it is the extreme of 

 flow, not the medium condition, that controls the cost of river regulation. It 

 is the floods and low waters that measure the cost. Any scheme of control 

 that is not based upon these Is worthless. This proposition need scarcely be 

 urged upon the experienced engineer. For himself he would never place any 

 real reliance upon forestry. Called in consultation, for example, in the problem 

 of protecting the city of Pittsburg from floods, he would be bound to take as 

 his measure of the problem the highest recorded flood on the river with a 

 good factor of safety on that, and then figure out by what methods artificial 

 reservoirs, levees, raising of grades, or clearing the river channel of artificial 

 obstructions he would obtain the desired relief. He would not dare, as the 

 physician in the case, to advise his patient that he could dispense with or 

 lessen in any degree the application of the remedies proposed, nor save one 

 dollar of the cost, by anything that might be done in reforesting the watershed' 

 of the rivers themselves. 



In like manner no engineer could honestly advise lowering in height by a 

 single inch the levees of the Mississippi, because of any possible application of 

 forestry to the watershed of that stream. And again he could not advise that 

 forestry development would lessen in any degree the cost of improving the rivers 

 for low-water navigation. Engineers fully understand their responsibility in 

 these matters. But great engineering projects can not be carried out without 

 money, and the people will not give the money unless convinced of the necessity 

 and wisdom of the plan proposed. So long as there is apparently some easier 

 and simpler plan, some panacea, no matter how nebulous or unproven, that 

 offers a way out without the expenditure of so much cold cash, they will be 

 backward in voting money, and the counsel of the engineer will be of no avail, 



Possibly the author is too positive in this opinion. He finds that, in one case 

 at least, the city of Williamsport, Pa., reputable engineers have advised refor- 

 estation of mountain slopes as a protection against floods. The statement of 

 " an eminent authority " was cited with approval to the effect that " four-fifths 

 of the precipitation is detained by the surface of the ground" under forest 

 Cftver. But here, as in all these assumptions, the rule applies only to the 

 average condition. The point is overlooked that in periods of heavy precipita- 

 tion the retentive capacity of the forest bed becomes exhausted. If the city of 

 Williamsport is relying upon this advice it is certainly laying up for itself a 

 season of repentance. 



