20 Canadian Forestry Journal. 



To mention the influence of the forest-htter (4), and the hold- 

 ing of the snow cover (5), suggests the third topic of my article. 



III. The influence of the forest upon the mechanical condition 

 and the erosion of the soil under its cover, is due simply to the im- 

 pedance offered to the fall of water by the forest litter. A heavy 

 rain falling unimpeded upon bare clay soil, almost invariably 

 packs it, puddles it, and injures its texture, its capacity to convey 

 water. Upon lighter soil the effect of the rainfall is less injurious, 

 but is always in the direction of compacting. The forest litter, 

 by breaking the force of the rain-drops, allows the soil to remain 

 open and friable. The tree roots also, penetrating the soil, form 

 channels for the entrance of water. Then, on sloping ground and 

 on hillsides, the loose litter retards the run-off, and allows the 

 water more time to percolate the soil. Thus, the rainfall is en- 

 couraged to become ground water, which, from the resistance 

 offered by the compact subsoil, flows slov^^ly down to lower levels, 

 and supplies springs and streams perennially. In the forest, with 

 its litter removed, the rainfall and the melted snow, instead of 

 becoming seepage water, largely run off the surface, denuding 

 the hillsides of serviceable soil, and stripping them to the bare 

 rock; while the rapid rush of surface water occasions destructive 

 freshets. Later in the season springs and streams dry up. 



Mr. J. W. Toumey, of the Washington Bureau of Forestry, 

 writes in the Year-book for 1903 on "The Relation of Forests to 

 Stream Flow." The following is a quotation from that article: 

 "In a careful study of the behavior of the stream flow on several 

 catchment areas in the San Bernardino Mountains, it has been 

 found that the effect of the forest in decreasing surface flow^ on 

 small catchment basins is enormous, as shown in the following 

 tables, where three well-timbered areas are compared with a 

 non-timbered one: — 



PRECIPITATION AND RUN-OFF DURING DECEMBER, 1903. 



Area of Condition Pre- Run-off Rnn-oft per- 



Catchment as to cipita- per square rentage of pre- 



basin. Cover. tion. mile. cipitation. 



Sq. miles. Inches. Acre-ft.* Percent 



' 70 Forested 19 36 > 



1 05 do 19 73 6 



1 47 do 19 70 6 



0.53 Non-forested. 13 312 40 



At the beginning of the rainy season, in early December, the 

 soil on all four of these basins was very dry as a result of the long 

 dry season. The accumulation of litter, duff, humus, and soil 



*640 acre-feet equal 12 inches of precipitation over a square tmle. 



