62 



velocity. As a result, the water passes rapidly over the surface, 

 and but little gets into the soil. When the soil is covered with 

 obstructions, such as are offered by a forest with its accumulation 

 of litter and vegetable growth, the rain which is not immediately 

 absorbed is checked in its flow over the surface. The water, 

 being held back, is finally taken up by the soil and thus prevented 

 from forming small rivulets through erosive action. 



The forest, in extending the time during which the rain 

 reaches the soil, in its effect upon local topography, and in sup- 

 plying a larger and better absorbing medium, must necessarily 

 have a profound influence in increasing the seepage run-off, and 

 in proportionately decreasing the surface flow. 



COMPARISON OF RUN-OFF FROM FORESTED AND NONFORESTED 



AREAS. 



There are so many complex conditions influencing the flow of 

 streams that it is extremely diflicult to determine the efl"ect of 

 forests on run-ofl" by the comparison of the discharge of streams 

 on forested and nonforested catchment areas. It is believed by 

 many that stream flow is so largely influenced by the amount 

 intensity, and character of the precipitation, the configuration and 

 area of the catchment basin, the character of the absorbing medi- 

 um and the underlying rocks, and the general climate, as well as 

 the forest itself, that we shall probably never be able to measure 

 quantitatively the influence of forests on the flow of streams by 

 the comparison of forested and nonforested regions. Catchment 

 areas differ so greatly in the features mentioned above that 

 our most conservative and able investigators have been forced 

 to the conclusion that " in respect to run-off, each stream is a law 

 unto itself." Although the above is probably in the main true, 

 yet, by the careful selection of small catchment basins for compari- 

 son, it appears that the influence of the forest in diminishing the 

 surface run-off can be determined with a fair degree of accuracy. 

 When the catchment areas compared are in the same region, are 

 influenced by the same or nearly the same climate and precipita- 

 tion and by the same storms, have approximately the same con- 

 figuration and area, and have a similar mineral soil and under- 

 lying rocks, the effect of these various factors on the run-off can 

 be ignored, and the differences in the behaviour of the stream flow 

 on the forested and nonforested areas can be assigned to the in- 

 fluence of the forest. 



In a careful study of the behaviour of the stream flow on several 

 small catchment areas in the San Bernardino Mountains, it has 

 been found that the effect of the forest in decreasing surface flow 

 on small catchment basins is enormous, as shown in the following 

 tables, where three well-timbered areas are compared with a non- 

 timbered one : 



