TIMBER; FORESTRY 119 



it has been possible for people interested in lumbering to 

 assert that clearing off the forests may do no harm to the 

 climate of the country. No one should be deceived by such 

 arguments, since it is a well-ascertained fact that the water 

 supply of any region is made far more irregular by the removal 

 of its forests. The annual rainfall may not be greatly changed, 

 but the rivers of a deforested country are likely to overflow 

 their banks after all heavy rains, and then almost disappear 

 during the driest part of the summer. A notable example of 

 this summer shrinkage in rivers is that of the Susquehanna ; 

 in 1816 it was estimated that this river delivered five hundred 

 million gallons per day at its season of minimum flow ; in 1874 

 this had shrunk to less than half the amount, although the 

 annual rainfall remained about the same. 1 Every one who has 

 lived in a prairie country knows that the run-off from grasslands 

 is so rapid that creeks which, during a dry time, consist merely 

 of a series of pools may be running bank-full after an hour or 

 two of heavy rain. From plowed land the run-off is even more 

 rapid than from grass-covered land. It is difficult to ascertain 

 just what proportion of the total rainfall is temporarily held 

 by the forest floor. On careful observation of a wooded basin 

 in the Cevennes Mountains in southern France, in which some- 

 what more than 50 per cent was forest, it was found that after 

 two days of heavy rain more than eight ninths of the total 

 rainfall was held by the soil; water so held may then run 

 away gradually. 2 



The action of forests in retaining water and slowly distrib- 

 uting it is largely due to the following causes : 



1. Snow melts gradually in the shade of forests, and so 

 the water derived from its thawing is given off little by little. 



2. Evaporation goes on slowly in the shade. 



3. The forest floor is often covered to a considerable depth 

 with a layer of highly absorbent material, such as decaying 



!See Fernow, "Forest Influences," Bulletin 7, Division of Forestry, 

 U. S. Dept. Agr., 1893. 

 2 Ibid. 



