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 1s 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 Cévennes 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 thea run 
away gradually.? 
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 
1 See Fernow, ‘Forest Influences,’’ Bulletin 7, Division of Forestry, 
U.S. Dept. Agr., 1893. 
2 Thid. 
