North Coast Watersheds Mapping

A Cooperative Effort of the California Department of Conservation and California Department of Forestry and Fire Protection

During the early 1980s, under the provisions of Section 208 of the Federal Water Pollution Control Act, the Environmental Protection Agency (EPA) funded a number of water protection projects that involved the geologic mapping of sensitive watersheds along California's north coast. Since then, the California Department of Conservation, California Geological Survey (CGS) has been contracted by the California Department of Forestry and Fire Protection (CDF) to produce maps for parts of Mendocino, Humboldt, and Del Norte counties, and other selected watersheds.

The long-range goals of the watersheds mapping projects are: 1) to retain productive forest soils, reduce sedimentation of north coast streams, and protect fish habitat; 2) to provide planning guides for timber harvesting and other forest management activities; 3) to comply with the Clean Water Goals of the Federal Water Pollution Control Act; and 4) to achieve the objectives in the state’s water quality control plans and other forest practice rules.

The maps provide information on the geology and geomorphic features affecting landslides for each of the watersheds studied. Physical characteristics that can be correlated to landslide potential, soil erosion potential and streambank erosion potential were mapped at a scale of 1:24,000. Sixty 7.5-minute quadrangles were completed by DMG between 1981 and 1985. Additional mapping in selected watersheds identified by CDF includes interpretative evaluations of relative slope stability for long-term planning purposes (see DMG Note 40).

Use of the Maps

Watershed maps are available to land managers, planners, and the public for uses aimed ultimately at the reduction of erosion and landsliding, and the enhancement of water quality.

The maps and legends will enable users to: 1) recognize and "flag" areas of potentially unstable ground, and 2) foresee and minimize potential problems in these areas. The maps should be most useful for identifying unstable and erosion-prone areas on a regional scale, and in the preparation of large scale, long-range management plans that use geologic information to minimize environmental impacts. The maps should not be used a substitute for on-the-ground site-specific studies (see DMG Note 45), but rather for identification of possible problem areas that need on-the-ground attention prior to development. Thus, identification of a landslide or geomorphic feature on the map does not preclude logging or other types of land development at that particular site.

Landslides

Photo of translational/rotational landslide — Translational/rotational landslide, Humboldt County. Photo by Tom Spittler.

Four categories of active and dormant landslides are depicted on the maps. They include debris slides, translational/rotational slides, earthflows, and debris flows and torrent tracks. Debris slide amphitheaters and slopes and inner gorges are not technically landslides, but features formed by landslides processes. These too are included on the watersheds maps. In some places, complex landsliding causes irregular ground surfaces that are indistinguishable or too small to map at the 1:24,000 scale. Such areas are depicted on the maps as disrupted ground or in those watersheds where relative slope stability maps exist, areas of extreme, high, moderate, and low relative stability are identified. Factors affecting landslide potential in these areas are described in DMG Note 50.

Availability

Digital versions of the maps can be obtained through the CGS Information Warehouse: Landslides.

You may also request GIS data and all PDFs as one download (277 megabyte .zip file)

Hard copies of the maps may be referenced at the CGS Library in Sacramento, or at the Eureka office (by appointment only). For office locations and contact information, go to Contact CGS.