Generally, the greater the number of stream crossings, the greater the number of sites where sediment can readily be delivered to channels. Fine-grained soils are particularly sensitive to such surface erosion. The erosion and transport of sediment from roads is exacerbated by the greatly reduced infiltration capacity of mineral soils on cut banks, running surfaces, and fill slopes, caused by compaction and the loss of organic horizons. Roads and skid trails also intercept and concentrate surface runoff so that it has more energy to erode even stable soils. Roads in areas with higher rainfall and snowmelt rates tend to exhibit higher levels of erosion than roads in drier areas.
Roads can also cause rapid mass movements and result in very large increases in sediment loads.
The negative effects of roads and skid trails can be moderated by laying them out to avoid the more sensitive sites, using appropriate road and drainage structure construction techniques, installing waterbars on non-permanent roads, and adopting a variety of other erosion control techniques. Roads include all hauling and in-block roads. Measure the total length of all roads in the watershed and divide by the total sub-basin area. The two most important factors in determining how much fine sediment will be delivered to streams from road running surfaces are the proximity of the road to the stream and the parent material on which the road was built. This indicator attempts to quantify this hazard.