8 rOEESTS, WOODS, AND TEEES 



remembered that when there occur abnormally heavy 

 precipitations or long-continued rains, the forest floor will 

 become saturated and be unable to absorb more water, so 

 that disastrous floods may occur even in the best forested 

 areas. The forest, however, plays an important part in 

 preventing a certain proportion of the otherwise inevitable 

 floods ; and its effect on the supply of water to springs is 

 undoubted. Forests are efficient to a high degree in pre- 

 venting erosion of the soil, formation of torrents, disastrous 

 floods, and the filling up of the beds of rivers with silt. 

 In channels filled with sediment even a slight rainfall may 

 cause a flood, hence the utility of the forest in keeping 

 streams and rivers deep and capable of carrying away 

 unusual quantities of rain. 



The effect of deforestation in increasing the number and 

 seriousness of floods was well shown by M. 0. Leighton (6) 

 in 1909, who proved that floods in the United States had 

 been increasing in most rivers, no other cause being dis- 

 cernible than the continuous felling of timber in the upper 

 part of their watersheds. 



It is the absorbent nature of the ground that determines 

 whether or not a larger or smaller proportion of the rainfall 

 and snow will run off directly into the river. A large 

 proportion is a flood. None of the conditions in the river 

 basins studied by Leighton appeared to vary, except the 

 surface vegetation, as the climate, topography, geology, etc., 

 remained unaltered. The variation in the surface vegetation 

 was the continuous reduction of the forest area by felling 

 timber in the river basins. The watersheds studied were 

 those of three tributaries of the Ohio Eiver in its upper 

 drainage area. During the last 20 to 30 years, there was 

 an increase of floods ; and when the variation in the annual 

 rainfall has been allowed for, Leighton's diagrams show that 

 the only factor that could have had any influence in 

 increasing the floods was the constant and rapid deforesta- 

 tion that had been carried on during the period in the 

 three river basins. 



Hall and Maxwell (7), who studied the conditions of 



