May 8, 1896.] 



SCIENCE. 



707 



of authority. This argument is, if I understand 

 it correctly, that in the Western mountains the 

 rlverflow is dependent on the accumulation of 

 winter snows ; that on the open ground these 

 snows are drifted, accumulated and packed to- 

 gether, whereby the melting of the snow is re- 

 tarded and the supply of available water pro- 

 longed ; that in forests the snow melts sooner, 

 because lying less thick ; that various other 

 causes, like mechanical obstruction to the snow 

 in reaching the ground, transpiration, greater 

 evaporation under trees, etc., reduce the avail- 

 able water supplies and hence that forests as 

 far as waterflow is concerned are an evil. This 

 deleterious effect, by the way, is argued almost 

 in the same breath with which the statement is 

 made that the forest growth in these mountains is 

 so open , casts so little shade, accumulates so small 

 amount of litter and offers so little obstruction 

 to sun and wind that its effect in shading and 

 protecting the soil and reducing evaporation 

 may be set down as nil. 



Now it is true that the rivers of the Sierra 

 rely for their supply mainly on the snow 

 waters, hence any conditions which preserve 

 and lengthen this supply, will influence the 

 quantity and continuity of the river flow. If, 

 therefore, the snow drifts melt more slowly and 

 at the same time give as much available water 

 in proportion to the amount of snow fallen, this 

 would be an advantage. The slow melting is 

 true, however, only for high altitudes above 

 timber line, which represent a comparatively 

 small area ; below timber line the snow drifts 

 are all gone long before midsummer, and it is 

 only with such as lie at similar altitudes, and 

 hence under similar temperature and wind con- 

 ditions, that the condition of the snow under 

 forest cover may be compared ; here even this 

 seeming advantage of the snow drift, the slower 

 melting, will be found not as great. 



But the very length of time during which 

 these snow masses are exposed to the other 

 dissipating influences, especially the ' dry air of 

 the mountains,' on which the writer dwells 

 with particular emphasis, is detrimental to the 

 amount which becomes available to the soil. It 

 is, therefore, by no means certain whether the 

 quantity of water delivered to the soil is in anj^ 

 relation to the time during which it is delivered. 



Knowing from tolerably reliable measurements 

 the enormous evaporative power of air, espec- 

 ially when in motion, with high velocities of 

 wind such as are common in high altitudes, we 

 have good reason to doubt this, although un- 

 doubtedly the drifting, and hence reduction of 

 exposed surface, reduces this loss somewhat. 



It would appear much more desirable to have 

 the snows melt quickly, provided their waters 

 have time and opportunity to sink into the soil 

 and away from the dissipating influences of dry 

 air and wind, which are bound to rob the exposed 

 drifts and leave less water for the soil. And here 

 we reach the most important lack in the writer's 

 argument and the most important claim of those 

 who argue an influence of forest cover on water- 

 How, namely, as to the manner in which the 

 rivers receive their water. 



Even if we grant, for argument's sake, the 

 unsubstantiated assertions of the writer, that 

 the forest cover on these mountains is too sparse 

 to exert any but deleterious influences with re- 

 gard to conservation of snows, a contradiction 

 in itself, he overlooks the most potent effect, 

 which even the stumps as well as all shrubs 

 and young growth have on the penetrability of 

 the soil for the water. 



He overlooks, as most writers on the subject 

 do, the fact that it is not so much the surface 

 drainage which reaches the rivers that forms 

 the desirable supply, as the subterraneiJn or 

 ground waters. Surface drainage means rapid 

 flow, high water stages, alternating with low 

 water, uneven distribution through the year. 

 Subdrainage means less excessive water stages, 

 more even, steady and persistent flow, for the 

 ground water reaches the river sometimes only 

 several years after it first sank into the soil, and 

 hence equalizes the effects of dry and wet sea- 

 sons while the surface waters are carried off at 

 once and are responsible for floods, followed bj' 

 low water. Anything, therefore, that tends to 

 change surface drainage into subdrainage is to 

 be encouraged. 



If there were, therefere, no other means by 

 which a forest cover acted as a preserver of 

 water supplies, the mere existence of the root 

 system, penetrating the soil in all directions 

 and facilitating percolation of the water, would 

 be beneficial. 



