PREDICTING DOWNSLOPE TRAVEL OF GRANITIC SEDIMENTS FROM FOREST ROADS IN IDAHO

The length and volume of granitic sediment deposits were measured annu ally over a 4.5-year period below 6.6 km of forest roads constructed o n headwater watersheds in the mountains of Idaho. Sediment deposits we re identified by source of runoff, location of the deposit terminus, a nd various site factors including descriptors of hillside sediment sto rage capacity. Prediction equations were developed using linear regres sion for travel distance of sediment originating from fill slopes, roc k drains, and culverts, with r(2) values ranging from 0.70 for fill sl opes to 0.91 for culverts. Cumulative volume of erosion and length of obstructions on the hillside were statistically significant variables in the equation for fills and rock drains; hillside gradient and runof f source area also appeared in the equation for culverts. Sensitivity analysis showed that the volume of erosion was the most sensitive vari able in all cases. A dimensionless relationship relating the volume of sediment storage on slopes to sediment travel distance was also devel oped. This study makes it possible for land managers to assess the ris k of sediment delivery to channels from alternative road designs, loca tions, and erosion control practices and provides a means to evaluate the cumulative effects of past road construction on sedimentation.