Evaluation of a geographic information system model of shallow landsliding in Redwood Creek, California

Abstract

The forested slopes of north coastal California drain to a dense network of streams that provide habitat for numerous threatened and endangered anadromous salmonid species listed under the federal Endangered Species Act. Land surface alterations in combination with a series of large storms in the region have increased the amount of sediment delivered to stream channels from fluvial and landslide erosion, contributing to the degradation of aquatic habitats. With concern for the protection of public trust resources, natural resource managers at Redwood National and State Parks (“the Park”) work to improve landuse practices on private timber and ranch lands upstream of the Park in the Redwood Creek watershed. Geographic information systems (GIS)-based, physical process driven models have been developed to delineate areas with a high potential for shallow landsliding. This project documents the application and evaluation of one such model, SHALSTAB, in the Redwood Creek watershed of northwestern California. Model results were interpreted in the context of existing shallow landslides. The utility of model output to Park resource managers was evaluated. SHALSTAB model results flagged 13% of the Redwood Creek watershed as having a high potential for shallow landsliding. Slopes in this category, including inner gorge slopes, were shown to capture 75% of the shallow landslides mapped within the watershed. Model output was field tested in the Lake Prairie Creek sub-watershed of upper Redwood Creek. Model results proved instructive to Park geologists involved in the timber harvest review process. Equipped with maps of model output, Park geologists prioritized timber harvest plans warranting field review and evaluated slopes of potential concern during pre-harvest field inspections. Rather than accepting model output at face value, Park geologists rely on their professional judgement to evaluate slope stability once on site, discussing conditions with foresters and state regulatory agencies, and recommending mitigation measures where warranted.