SURFICIAL GEOLOGIC MAPS HELP COUNTIES UNDERSTAND GEOLOGIC HAZARDS IN IDAHO

The Idaho Geological Survey’s long-term geologic mapping plan is designed to serve the natural-resource base and the growing population of Idaho. However, utilization of geologic mapping by counties and cities has lagged even as maps have become available. During the 1990s, this underutilization began to change as more counties and cities began to use GIS data for their planning and decision making. The Idaho Geologic Mapping Advisory Committee designated the Clearwater corridor as a high map priority and the IGS received a STATEMAP award to map the area which includes parts of Nez Perce County. The IGS cooperated with Nez Perce County to identify critical planning issues and priority areas. Because of recurrent landslides, the county came to understand the utility of basic geologic mapping to planning, zoning, and permitting.

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 county’s 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 county’s 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.