CHAPARRAL COVER, RUN-OFF, AND EROSION 



By E. N. Munns 



Forest Bxaminer, U. S. Forest Service 



In September of 1919, two fires occurred on the Angeles Forest 

 which burned some 150,000 acres of brush and forest land as severely 

 as any fire of which we have any record. The two fires occurred 

 simultaneously and were spread by a high north wind much more 

 rapidly than any fires of which we have record. This wind caused the 

 fires to burn much more intensively than the usual chaparral fire and, 

 as a corollary, much more severely. It is known that in one place the 

 fire swept over five miles in twelve hours, traveling uphill and downhill 

 at practically equal rates, burning almost continuously across a broad 

 front roughly three miles in width. Both fires were put out by a 

 rain of fairly high intensity which amounted to several inches in three 

 days. On Sister Elsie Peak, elevation 5,000 feet, a recording rain 

 gauge registered 4.50 inches for the storm, though there was much 

 less rain over the entire area. 



The present discussion is limited to four areas known as San Dimas, 

 Pacoima, Little Tejunga, and the Dalton Canyons. All these drainage 

 basins were completely burned over by the two fires, wath the excep- 

 tion of San Dimas, in which 40 per cent of the cover at the lower 

 elevations on the south and west slopes was destroyed. Cover con- 

 ditions on these areas prior to the fire have been described in detail 

 elsewhere but a resume of conditions as existed is necessary to under- 

 stand the changed conditions. 



The soil of the San Gabriel J.Iountains is derived from a rapidly 

 decomposing granite which outcrops in many places, and the average 

 soil depth is over two feet for the entire area where it occurs. In 

 the Tejunga-Pacoima region, past erosion has exposed a sandstone 

 conglomerate which covers approximately a fourth of the Pacoima 

 and a third of the Tejunga drainage. Here the brush cover was not 

 as dense nor as complete as elsewhere and erosion was widespread and 

 severe. There is very little permeable surface soil, but the sandstone 

 itself is probably more or less porous and absorbs considerable moisture 

 in a rain of long duration, though a heavy rain undoubtedly would 

 immediately run ofT without much absorption. 

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