Figure 5. — The debris 

 avalanche scoured the 

 bottom of Watershed 3 

 to bedrock. The slide 

 obliterated the sedi- 

 ment dam (formerly in 

 the channel bottom) and 

 splashed mud on top of 

 the storage rain gage 

 tower ( see arrow) . 



water contents. Debris avalanches usually leave a discernible elongated scar to bedrock 

 at the slide origin and often exhibit a characteristic downslope slide path. The lower 

 jammer road in Watershed 3 was constructed through the old slide area without taking 

 special precautions (fig. 2). In April 1965, a combination of rainfall and snowmelt 

 generated a massive failure of the road fill material at the site of the old landslide 

 (fig. 4). The slide scoured the entire length of the channel to bedrock in Watershed 3 

 and destroyed the sediment dam (fig. 5). Postslide measurements indicated that approx- 

 imately 6,030 cubic feet of sediment moved down the channel. 



Total Erosion 



The total surface erosion for the three watersheds for each sampling period is 

 shown in table 3. Data for all three watersheds (before sediment dam 3 was destroyed) 

 are included in this total because natural sediment barriers are commonly found on 

 slopes in this vicinity. Surface erosion for the entire 6-year study period totaled 

 2,413.5 cubic feet. The 1965 mass erosion event amounted to an additional 6,030 cubic 

 feet of sediment, which, added to the surface erosion, totaled 8,443.5 cubic feet of 

 erosion from roads for the 6-year study period. Thus, about 30 percent of the soil 

 loss could be attributed to surface erosion and the remainder to mass erosion. Actually, 

 the total percentage of surface erosion might be greater by a few percent because some 

 erosion undoubtedly continued in Watershed 3 after the destruction of the sediment dam. 



The effects of road construction and logging on sediment movement can best be 

 appreciated by comparing the rates generated by these uses to the rates for undisturbed 

 lands. Sediment dams were used to determine sediment yields on undisturbed, perennial 

 watersheds in the immediate vicinity. These included the Oompaul , Hamilton, Tailholt, 

 and Circle End drainages, which are 740, 460, 1,625, and 930 acres in size, respectively. 



7 



