Sediment Production from Forest Roads in the Central Sierra Nevada, California

Unpaved roads are a major source of sediment in many forested landscapes. For the Sierra Nevada of California there are few data on road erosion rates or the factors that control road sediment production. This information is urgently needed to establish TMDLs and evaluate cumulative effects. Annual sediment production from 139 road segments was measured over three wet seasons using sediment fences. The first wet season had 1290 mm of precipitation as compared to 890 and 1057 mm in the second and third wet seasons, respectively. More importantly, the second and third wet seasons were dominated by colder storms with much of the precipitation falling as snow. Hence the annual erosivity (EA) in the first wet season was 847 MJ mm ha-1 hr-1, which was nearly twice the values from the second and third wet seasons. The mean sediment production rate from native surface roads in the first wet season was 0.64 kg m-2 as compared to 0.17 and 0.20 kg m-2 in the second and third wet seasons, respectively. Rocked roads produced only 0.01-0.03 kg m-2 yr-1. Recently-graded native surface roads produced twice as much sediment per unit storm energy as ungraded roads. The product of road surface area and road slope explained 40% of the variability in sediment production from native surface roads. Adding EA and a dummy variable for grading increased the overall R2 to 0.54. Road segments draining soils with rock outcrops or particularly shallow soils produced more than twice as much sediment as roads on or adjacent to all other soil types, and this difference is attributed to the higher runoff rates associated with the interception of shallow subsurface stormflow. The much lower erosion rates in the second and third wet seasons can be attributed to the lower annual erosivities and the role of the snow cover in reducing rainsplash and overland flow velocities, and possibly attenuating high-intensity bursts of rainfall. The results emphasize the variability of road erosion rates, the sensitivity of road erosion rates to site and climatic factors, and the difficulty of accurately predicting sediment production rates.