303 REPORT OF the; forestry committee 



filled entirely or partially with foreign material would hold large quantities of 

 water while the denuded slopes deliver the storm water almost as fast as it falls. 

 Of all vegetable covers forests are most efficient in preventing the slopes 

 from eroding and the beds of streams from filling with silt. Even on very per- 

 meable soils where their effect upon the underground seepage of water may be 

 of secondary importance, they are necessary as protection against erosion. 



FOEEST AND FLOODS 



THE observations upon the effect of the individual factors affecting stream- 

 flow are also confirmed by records and observations on the behavior of 

 the streams themselves. 

 The available observations upon the behavior of streams in this country 

 and abroad have established the following facts : 



1. The total discharge of large rivers depends upon climate, precipitation, 

 and evaporation. The observed fluctuation in the total amount of water carried 

 by rivers during a long period of years depends upon climatic cycles of wet and 

 dry years. 



2. The regularity of flow of rivers and streams throughout the year depends 

 upon the storage capacity of the watershed, which feeds the stored water to 

 the streams during the summer through underground seepage and by springs. 

 In winter the rivers are fed directly by precipitation, which reaches them chiefly 

 as surface run-off. 



3. Among the factors, such as climate and character of the soil, which affect 

 the storage capacity of a watershed, and therefore the regularity of streamflow, 

 the forest plays an important part, especially on impermeable soils. The mean low 

 stages as well as the moderately high stages in the rivers depend upon the extent 

 of forest cover on the watersheds. The forest tends to equalize the flow through- 

 out the year by making the low stages higher and the high stages lower. 



4. Floods which are produced by exceptional meteorological conditions can 

 not be prevented by forests, but without their mitigating influence the floods are 

 more severe and destructive. 



CONCITJSIONS 



THE facts brought out in this report clearly show that there is an intimate 

 relation between the forests, the climate, and the regularity of the flow 

 of water in the streams. 

 What should then be the practical deductions from this established relation? 

 How large should be the forest area in a country so as to prevent any bad effects 

 of the destruction of forests upon climate and the water supply in rivers? 



There are no accurate means of determining the extent of forest land neces- 

 sary for the regulation of streamflow and the protection of the soil against erosion. 

 From the study of conditions, however, existing in other countries it may be 

 inferred that, in order not to disturb the natural balance, the proportion of forest 

 land to other kinds of land must be not less than from one-fifth to one-third of 

 the total area of the country. 



