Impacts of a Wildfire on Road Surface Erosion and Road-stream Connectivity

Unpaved roads are a major hydrologic disturbance and source of sediment in forested watersheds. High- and moderate-severity wildfires should increase road sediment production due to the increased surface runoff from upslope, and increase road-stream connectivity due to the increased runoff and decrease in downslope surface roughness, but there has been no study of these interactions. Hence the goal of this study was to quantify how fire severity affects road surface erosion features and road-stream connectivity. Detailed data were collected one year after the High Park wildfire along 6.8 km of an unpaved road west of Fort Collins, Colorado. Road segments below areas burned at high and moderate severity had significantly more rills than segments below areas burned at low severity. Road segment slope was an increasingly important control on the proportion of segment length with rills with increasing burn severity, and this is consistent with the known increases in surface runoff. Flatter road segments acted as barriers that accumulated sediment eroded from the hillslopes above the road. All of the road segments in areas burned at high and moderate severity had drainage features that extended to a stream, and 78% of the segments in areas burned at low severity were connected. These exceptionally high rates of road-stream connectivity are attributed to the increased runoff from upslope, the concentration of that runoff by the road surface to a single drainage point, and the reduced infiltration and trapping capacity of the burned area below the road. The results show the need to either outslope the roads or increase the frequency of constructed drainage features after wildfires, particularly for the steeper road segments in areas burned at high or moderate severity.