74 PROCEEDINGS OF THE 



stream gauging stations. It is proposed to divert the 

 water flowing from a number of these small mountain 

 basins which are situated on the northerly slope of the 

 San Bernardino Range by means of gravity canals and 

 tunnels to the southern side of the range and into the 

 San Bernardino Valley. This Arrowhead Reservoir 

 Company has placed its hydrographic data at the 

 disposal of the Bureau of Forestry, which organiza- 

 tion made a forest study in connection therewith. The 

 data that is presented by Professor Tourney is perhaps 

 the most precise and definite information on the sub- 

 ject of related stream flow to forest cover that we 

 have so far been favored with in the West. His 

 conclusions, while they were to be expected, are grati- 

 fying in their definiteness. We can do no better than 

 to quote from Professor Tourney in extenso : 



"Because rainfall is most abundant where forests 

 grow, many believe that forests exert an important 

 influence on the amount of precipitation. A more 

 reasonable inference, however, is that rainfall is the 

 great factor in controlling the distribution and density 

 of forests. 



"Precipitation occurs whenever the air is suddenly 

 cooled below the dew-point. The most effective cause 

 of this is the expansion of air on ascending. This 

 upward movement is caused very largely by cyclonic 

 storms. Whether forests have any appreciable effect 

 in cooling the air to below the dew-point is uncertain. 

 From the known effect of forests on the temperature 

 and relative humidity of the air, it is reasonable to 

 infer that they may have some effect, at least to a 

 small degree, and consequently that they have some 

 influence in increasing precipitation. The present evi- 

 dence, however, derived from many series of observa- 

 tions conducted in Europe and elsewhere, is so con- 



