DISCUSSION AND CONCLUSIONS 



Total sediment production per unit area of road prism increased an average of 770 

 times for the 6-year study period. At present, the impact such an increase will have 

 on the important downstream salmon spawning resources is undefined; however, there are 

 indications that this level of impact is intolerable (Richards 1963). Short of forbid- 

 ding all road construction, our findings emphasize the need for: (a) More careful 

 planning to minimize the mileage of road construction, including consideration of 

 logging systems that require fewer roads; and (b) diligent location, design, construc- 

 tion, and maintenance to minimize erosion on the roads that are built. 



Surface erosion following road construction on steep, highly erodible batholith 

 lands, such as those found in Deep Creek, decreases rapidly with time. About 85 percent 

 of the erosion occurs during the first year after construction. This emphasizes that 

 (a) measures to control surface erosion must be applied as soon after construction as 

 possible to be effective; and (b) reseeding alone, as was carried out on Deep Creek, 

 is not the complete answer because vegetation is slow to respond. 



Observations suggest that most of the high initial surface erosion is actually the 

 result of erosion on exposed road fills (fig. 8). Considerable data indicate that 

 erosion on granitic road fills can be greatly reduced by stabilization measures such 

 as the treatments listed in table 5. Note that erosion rates on granitic road fills 

 can be reduced up to 99 percent by such stabilization treatments. The largest propor- 

 tion of surface erosion occurs during the first year and is generally the result of 

 erosion on fill slopes; therefore, it appears to be possible to greatly reduce sedi- 

 mentation caused by surface erosion on roads using these fill slope stabilization 

 treatments . 



After about 3 years, the extremely high initial sediment yields in Deep Creek had 

 dropped dramatically but still averaged about 50 times greater than did sediment 

 production on undisturbed lands. The erosion control treatments on road fills listed 

 in table 5 and water control measures on the road tread (e.g., culverts, etc.) should 

 help to reduce high, long-term sedimentation rates. However, it is not likely that 

 sediment yields will drop to the levels expected on undisturbed lands because the cut 

 slopes remain active sediment sources and, at present, practical erosion control 

 measures are not feasible. 



Even though erosion is occurring within a road prism it need not necessarily 

 increase sediment yields at some downstream point. Results of this study reinforce 

 those of earlier studies that showed the effectiveness of barriers (e.g., down logs, 

 branches, etc.) in inhibiting the downslope movement of coarse granitic sediments. 



Major impacts can still occur from mass erosion after sedimentation from surface 

 erosion has dropped to a low level and (for all practical purposes) a road is 

 considered relatively stable. In Deep Creek, a single storm event resulted in a road 

 fill failure that accounted for about 70 percent of the total sediment production for 



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