RELATION OF FOREST COVER TO 

 STREAM FLOW 



BY 

 J. B. LIPPINCOTT 



Supervising Engineer, United States Eeelamation Service 



'T'HE relation of rainfall to run-oflf is very uncertain, 

 depending upon the nature of the storms, whether 

 gentle showers or violent rains; the steepness of the 

 drainage basin and its covering, and whether the pre- 

 cipitation is snow or rain. It has been found that in 

 the districts where the forest cover is small the output 

 of the basin occurs in violent floods of short duration. 

 Because these floods are violent, and of large volume, 

 and owing to the fact that the soil of the drainage 

 basins is not hold together by a network of roots, ex- 

 tensive erosions occur in these barren basins and the 

 stream carries much silt in suspension. Where the 

 basin is covered by forest, the mat of twigs and leaves 

 which covers the ground is an absorbent sponge, 

 retaining in itself large quantities of water and pre- 

 venting evaporation from the underlying soil. This 

 permits of a holding back of the floods and the gradual 

 draining off of the water, thus largely accomplishing 

 the purpose of Regulating reservoirs. 



A striking example of the output of a barren, tree- 

 less, drainage basin is shown in the case of Queen 

 Creek, Arizona, for the year 1896. This stream dis- 

 charges only in violent freshets, recurring usually as 

 great flood-waves, subsiding almost as rapidly as they 

 arise. By making from two to three current-meter 

 measurements of each of these freshets, and keeping 



