Post-treatment erosion of decommissioned forest road stream crossings

Forest road decommissioning projects have increased in number over the last two decades. The goal of this study is to provide information which can help to focus restoration efforts on the most cost-effective use of technique and treatment. To pursue this goal, I examined the strength of several variables which may control erosion rates, and evaluated the relative importance of the type of erosion each variable affects. This study explores the relationship between the physical characteristics of twenty excavated forest road stream crossings and the post-excavation erosion at each of those crossings. The study areas were located in northwest California, at sites within Headwaters Reserve, Six Rivers National Forest, and Klamath National Forest. Channel incision erosion contributed the greatest portion, 93 percent, of total post-treatment crossing erosion. In 16 of the 20 study crossings, channel incision accounted for all of the total post-treatment erosion. Post-treatment erosion from excavated banks contributed approximately seven percent of the total post-treatment erosion volume. The most significant independent variable in explaining the depth of post-treatment channel incision was watershed size. Watershed area explained 79 percent of the variation in channel incision depth within the Wildcat Formation, and 90 percent of the variation in observed post-treatment incision in two sandstone formations (Wildcat and Yager). Similarly, stream power was found to explain 65 percent of the variation in channel incision in the Wildcat Formation, and 56 percent of the variability in channel incision depth in the two sandstone formations.