Factors Contributing to Landslide Activity in the Winter of 1995-96, Clearwater County Near Orofino, Idaho
Significant landslide activity occurred in Clearwater County, Idoho in November 1995 and February-May 1996. Mass wasting in the study area consisted of debris slids and earthflows triggered by rain-on-snow weathr events. It is important to determine what factors contribute to landsliding in this area so that reliable prediction can reduce the destruction of property. Through field observation and aerial photo analysis, the factors contributing to landslides were studied. The objectives included a study of the geology, soils, aspect, slope gradient, vegetation and slope position related to each slide. Also a goal was to assess the role of land use in triggering landslides, analyze climatic conditions during precipitation events, and to create a landslide hazard map of the Orofino, Idaho area.
Thirty-two landslides were identified in the study area. Forty-one percent of landslides were produces at sites impacted by roads, and involved either the road prism or artificial channel areas. Most of the landslides were associated with roads, and occurred at lower slope gradients than those found in forested areas. Most landslides originated on slope gradients of 30-50%. Forested slopes account for the steepest gradients, and frequently have a northerly aspect. Landslide activity occurred most frequently in soils with a basalt parent material component. The largest volume landslides occurred in forested areas in which a geologic contact between basalt and metamorphic rocks created springs. Within some areas of the study region the bedrock geology has a greater role in landsliding than land use. Historically, the Souther Oscillation has been positive in the winter months when large rain-on-snow weather events have caused flooding in the study area.
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