Evaluating Large-Scale Long-Term Forest Road Restoration Effects on Public Lands in the Northwestern United States

Forest roads are a primary source of management-related chronic fine sediment to mountain streams. In the early 1990s, the Northwest Forest Plan (NWFP) directed major changes to the focus of public land management across the northwestern US: from production of timber as a primary theme to restoration of ecosystem function as paramount. Efforts shifted to retaining diversity of forests and improving aquatic habitat. A key practice was decommissioning or improving roads to reduce sediment inputs from surface erosion and shallow landslides. We evaluated the effect of road sediment reduction activities over the extent of the NWFP by looking at modeled sediment production and landslide risk changes between the 1993 and 2019 road networks. Key questions are about where and how much sediment delivery and landslide risk changed and how road selection decisions affected outcomes. Surface erosion and delivery values and shallow landslide risk indices were calculated for the 6th code Hydrologic Units in the NWFP area. Surface erosion and delivery were estimated using the GRAIP-Lite model applied to the federal road network, which accounts for road slope, road surfacing, traffic/maintenance level, and proximity to streams. Shallow landslide risk was calculated as a convolution integral of road length with the cohesion value required for stability based on the infinite slope equation. Detailed spatial modeling provided opportunities to diagnose contributions to changes and efficiency of segment selection. Decommissioning led to a 6.6% decrease in connected road length and 4% decrease in sediment delivery. The majority, 96%, of the watersheds in the Northwest Forest Plan area showed <0.5 Mg/km/yr decrease in sediment yield, and 4% showed greater decreases, with a maximum decrease of 3.4 Mg/km/yr in one watershed. The sediment delivery ratio increased in many watersheds where overall delivery decreased, indicating that roads with lower connectivity were selected for decommissioning. Substantial decreases in landslide risk were seen in watersheds with extensive initial roading in high-risk areas. In areas where restoration work was focused, large changes in road sediment delivery and landslide risk were achieved. The remaining road network was more connected to the stream network but located on more stable slopes.