. Effect of logging roads on sediment production rates in the Idaho Batholith. 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 19
. Effect of logging roads on sediment production rates in the Idaho Batholith. 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, 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, 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 lan
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