82 FOREST LANDS FOR THE PROTECTION OF WATERSHEDS. 



ited again, would in part be carried out to sea, and the bottom lands, unless 

 protected, would be gradually eaten away. 



In addition to the four main propositions discussed above, a few subordinate 

 features of the question will now be considered. 



A feature of the Forestry Service which is generally overlooked is the possible 

 effect of culture upon the bed of humus, so much relied upon in these discussions 

 to prove the restraining action of forests upon run-off. Mr. Piuchot, in his 

 statement to the Judiciary Committee, said : 



" The effect of a forest on a steep slope is to cover that slope with leaves, 

 rotten and half rotten sticks, and other mechanical obstructions, which prevent 

 the water from running below as rapidly as it would otherwise." 



It is understood that the forest policy is to keep this litter cleared up as a 

 measure of fire protection, and one frequently sees in articles on forestry photo- 

 graphs of the typical forest culture in which the ground is thoroughly cleaned 

 up. The result must be to diminish proportionately the retentive action of the 

 forest bed and to increase its liability to erosion. In the light of the foregoing 

 discussion fire protection is of much greater importance than the retentive effect 

 of the forest bed on the run-off. The remarkable degree to which the forest bed 

 will dry out in prolonged drought, making it one vast tinder box, supports this 

 conclusion, and is another proof of the extreme desiccating effect of forest 

 growth upon the soil. 



It often escapes attention, except with those who are in the woods a great 

 deal, that the water establishes little channels through the debris where the 

 latter is of long accumulation and somewhat permanent in character. Such 

 debris does not in reality offer so great an obstruction to flow as one would 

 suppose, and as would be the case if its condition underwent frequent change. 



The statement is constantly met that forests are very efficacious in the pro- 

 tection of river banks from undermining and steep slopes from sliding. . The 

 exact reverse is the case. As every river engineer knows nothing is more disas- 

 trous to a river bank on an alluvial stream than heavy trees. This is due partly 

 to the great weight, but in large part to the swaying effect- of the wind and the 

 enormous leverage of the long trunks which pry up the ground and facilitate 

 the tendency to undermining. One of the regular policies of river control is to 

 cut down these trees for a distance back from the edge of the bank wherever 

 complications with private ownership do not prevent. Snags and driftwood in 

 the channels have always been among the most serious obstacles to navigation 

 on streams flowing between forest-covered banks. Likewise where railroad or 

 highway grading cuts the skin of unstable mountain slopes, the presence of 

 large trees immediately above tends powerfully to loosen the ground and cause 

 it to slide ; and in such cases it is necessary to cut down the timber. Far better 

 than forest trees on river banks are thick growths of willow, alder, or any of 

 the smn Her close-growing shrubs; and on side hill slopes either such shrubbery 

 or a good turf. 



In the current discussion a great deal is made of the fact that mountain 

 slopes are "quick spilling," the deduction being that they therefore are more 

 productive of floods. This is quite contrary to the fact. It is perfectly true 

 that more rain falls on the hills than on the lowlands, that a greater percentage 

 of rainfall runs off from steep than from flat slopes, and that it runs off more 

 rapidly; but it does not follow at all that these conditions produce greater 

 floods. A mountain stream carries off the water within its banks a great deal 

 faster and more safely than a similar stream in the lowlands. The banks are 

 almost always stable and the bottoms rocky or composed of heavy gravel or 

 lowlders; in fact, floods do less harm on such streams than on any others. In 

 the lowland, where the streams have smaller slopes and unstable banks, much 

 smaller run-off produces greater floods and more destruction. Moreover; nature 

 to a large degree adapts streams to the work required of them. The channels 



The following testimony before the JJoard of Consulting Engineers, Panama 

 Canal, is to the point (Report, p. 329) : 



Question by Mr. Welcker : Mr. Chairman, I would like to ask if Mr. Dauchy 

 thinks that vegetation prevents the sliding? 



Mr. DAUCHY. My experience has been the reverse ; I have stopped sliding hills 

 by cutting off the vegetation. The weight of the timber on a sliding slope aids 

 materially to assist the sliding. 



Mr. WELCKER. Does not the vegetation diminish it? 



Mr. DAUCHY. If you could get a grass-covered slope it would help to diminish it. 





