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 effect 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 differing 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 
