First-year erosion responses following stream channel crossing fill removal in Redwood National and State Parks, Northwestern California

Road restoration techniques have been proven effective in reducing sediment delivery over the span of decades throughout the northwestern United States. However, few studies have been conducted to test the effectiveness of post-treatment erosion control methods and the short-term progression of erosion on removed roads, especially in the first year. This study examines twenty road stream crossings by surveying channel adjustments and determining rilling and gullying delivery volumes, channel substrate changes, and the role of large wood and rock placed in reconstructed channels. All measurements were conducted in the summer of 2006, and again in the summer of 2007 at Redwood National and State Parks, where a variety of erosion control measures are used on removed roads. A one-way analysis of variance found that erosion was higher at crossings in the first year following removal than at older crossings (P = 0.039). Eight first year crossings contributed a total of 61 cubic meters of sediment, while the second- and third-year crossings contributed no net sediment. Ninety-three percent of erosion from crossings, 57 cubic meters, could be attributed to channel incision and bank erosion. The remaining seven percent, 4 cubic meters, was tied to rilling, gullying, and side-slope failures. The first storms of the year were most important in mobilizing sediment. The rate of coarsening of channel substrate was significantly higher for crossings in the first winter after removal. Large wood ( 10cm diameter) placement in channels had no significant impact on channel incision, but placement along banks was observed to be beneficial in armoring against bank erosion. In many cases, the lack of availability of on-site materials, specifically organic matter and boulders or cobbles, can limit the level of mulching or armoring that can be applied. Nonetheless, current treatment techniques are effective in reducing sediment mobilization following forest road removal.