INTRODUCTION 



The Idaho Batholith is a 16 ,000-square-mile expanse o£ granitic rocks located in 

 central Idaho (fig. 1). Much of the region is characterized by steep slopes and 

 shallow, coarse- textured soils overlying granitic bedrock. Soils of this type have 

 been shown to be extremely erodible (Anderson 1954; Andre and Anderson 1961) . 



The potential for erosion following road construction on side slopes in the Batho- 

 lith is greatly increased within and sometimes beyond the road prism. Some factors 

 causing the increase are: interruption of subsurface flow; removal of vegetative cover; 

 destruction of natural soil structure; cut and fill slopes, which necessarily exceed 

 the original slope gradient; and decreased infiltration rates on portions of the road. 

 Megahan and Kidd (1972) reported on a 6-year study of erosion from secondary logging 

 roads in the Zena Creek area of the Payette National Forest. Erosion on the area 

 disturbed by road construction averaged 770 times greater than that occurring on nearby 

 undisturbed watersheds. 



In general, the erosion potential probably is increased more on the fill portions 

 of a road than on the cut portions. Hydrologic analysis of the Zena Creek logging 

 study area conducted by the Payette National Forest in 1966 showed that surface erosion 

 on fills was an estimated 1.75 times that on road cuts. Evaluations also showed that 

 material moved by mass erosion (landslides) amounted to almost as much as that resulting 

 from surface erosion. Most landslides in the Zena Creek area occurred in the fill 

 portions of roads. 



Bethlahmy and Kidd (1966) reported that grass seeding, coupled with straw mulch, 

 bound in place by erosion net, reduced surface erosion on a granitic road fill by about 

 98 percent the first year. However, the Payette National Forest hydrologic analysis on 

 Zena Creek showed that an established grass cover did not prevent mass erosion of 

 road fill surfaces. In some cases, as soon as the fill became sufficiently saturated 

 with moisture, it "melted" and flowed down the hillside like wet concrete. The con- 

 clusion was that grass roots cannot bind a massive sandy fill together if the fill is 

 standing at greater than its natural angle of repose. 



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