Ch. 19] THE PROBLEM 337 



ered by a thin mantle of coarse, rocky soils. Intensive urban and 

 suburban development has encroached upon, and in some places com- 

 pletely occupied, the debris-cone areas at the mouths of the canyons. 

 In those areas debris flows may be large and may cause great damage 

 (Dodge, 1947). 



In Utah a somewhat similar situation exists along the west front 

 of the Wasatch Mountains. In this area debris flows contain a greater 



Fig. 1. La Crescenta area, San Gabriel Mountains. The flood that moved these 



boulders onto the pavement came from Dunsmuir Canyon, with a drainage area 



of about 500 acres. The boulder in the foreground was measured and weight 



calculated to be 40 tons. 



percentage of fine alluvium and have been described as mud and rock 

 flows. The development in the debris areas, being mostly rural in 

 character, is less intensive than that in Los Angeles County, and 

 damage caused by debris flows is proportionately smaller (Woolley, 

 1946; Alter, 1930). 



In northern California, on tributaries of the Sacramento River, the 

 debris problem is entirely man-made. Hydraulic mining operations, 

 which began at the time of the gold rush, resulted in the washing of 

 huge quantities of debris down to the principal streams. This material, 

 which was carried out to the valley floor by subsequent floods, filled 

 the river channels and caused flood overflow of adjacent lands (Par- 

 sons, 1947) . 



