INFLUENCE OF FORESTS ON CLIMATE 9 



the watersheds in the Appalachian Mountains, confirm 

 Leighton's conclusions. These investigators attribute the 

 great increase in the frequency and duration of the floods 

 in the rivers originating in this region, to the clearing away 

 of the forests on the mountain watersheds of the streams. 

 They strongly urge the preservation of the forests around 

 the headwaters of the rivers. Their researches brought out 

 the interesting fact that the increased run-off" due to forest 

 clearance diminished the evaporation over the watershed, 

 and in consequence lessened the local rainfall. In other 

 words, the rivers were found to discharge more water, 

 though the rainfall over the watershed had decreased. This 

 result, however, was to be expected, because if, as already 

 shown, the presence of forests increases local rainfall, then 

 their clearance will diminish it. 



Forest cover has been lately shown to have a distinct and 

 measurable eff'ect upon navigable streams in Pennsylvania. 

 This question had to be decided by the U.S. Geological 

 Survey before the Forest Service could purchase under the 

 Weeks Law any land in the White Mountains. The report 

 drawn up by Mr. Leighton (8) gives the result of his 

 observations on two small adjoining watersheds in this 

 region, precisely similar in area, shape, aspect, and geology, 

 but diff'ering in vegetation. One watershed was covered 

 with virgin forest ; the other, which had been cleared of 

 all its timber and burnt over subsequently, bore only a 

 cover of brushwood. Measurements of precipitation over 

 the areas and of the run-off of the respective streams show 

 that not only was the snow held better in the forested area, 

 but that during a period of 17 days in April, including 

 three extended storms, the run-off of the stream in the 

 deforested area was a comparative flood, practically double 

 that of the stream flowing through the forested area. 



The action of forests in diminishing floods of rivers is 

 not universally admitted. The observations of E. V. 

 Oppokov (9) on the relation of the river Dnieper to rainfall, 

 etc., from 1876 to 1908, seem to controvert Leighton ; but 

 are probably explained by the difference between the 



