SURFICIAL GEOLOGIC MAPS HELP COUNTIES UNDERSTAND GEOLOGIC HAZARDS IN IDAHO
The example we present is the use of a surficial geologic map in the western part of Nez Perce County near the city of Lewiston, which is located in northern Idaho near the boundary between the Columbia Plateau and the Northern Rocky Mountains. The selected area of the surficial geologic map is representative of the physiography and geology of the lower Clearwater River and U.S. Highway 12 corridor. The Clearwater River valley is steep-sided and landslides are common. The surficial geologic map shows units that characterize geomorphic processes and their potential as geologic hazards.
The countys need to is to delineate geologic-hazard areas that require site-specific geotechnical studies. The surficial geologic map is vital information; however, county decision makers are unable to translate the geologic units to practical engineering categories. The countys geotechnical contractor interpreted the engineering properties and material characteristics of the geologic units and produced a derivative map showing geotechnical terrain units(GTU). Each GTU includes a description of its capabilities for the following categories: Slope, ground water, erosion, soils, earthwork, roadways, foundations, septic systems, and site-specific study. This example shows that geologic mapping under the National Cooperative Geologic Mapping Program is an important foundation for county planning, zoning, and permitting. The collaboration between the IGS and Nez Perce County is serving as a model for increased use of geologic mapping elsewhere in Idaho.