Detailed air photo analysis coupled with field mapping provides the most consistent and accurate identification of areas prone to or undergoing deep-seated landsliding. Certain geologic units are notorious “bad actors” because of their rapid weathering characteristics, accelerated by regionally high annual precipitation and a lack of influence from Pleistocene glaciation. The Lincoln Creek Formation (LCF), an Eocene to Oligocene marine sedimentary tuffaceous siltstone and silty sandstone underlying much of southwest Washington, is such a unit. The LCF weathers to silts and clays with soft to very stiff consistency and typical plastic indices ranging from 20 to 50. Observed residual shear strengths of these materials can range from f=13° to 28° and c=O kPa.

Landslide densities in areas underlain by LCF commonly exceed 10/km², with large coalescing slides covering up to 0.3 km². High groundwater conditions persist throughout much of the year in residual soils of the LCF because of their poorly-drained nature and the high average annual precipitation. This may contribute to the fact that many of the landslides remain active for years at a time. Instrumented landslides that exhibit periodic movement show strong correlations of piezometric surface fluctuations, with recognizable lag times, to precipitation events.

The high sensitivity of hillslope stability to ground water changes suggests that much of the forested terrain underlain by LCF is also highly sensitive to decreases in evapo-transpiration brought on by the effects of timber harvest. Harvest plans should incorporate detailed, sub-basin scale mapping of landslide areas with estimates of background and accelerated sediment input to stream channels. In areas underlain by large numbers of deep-seated (and other) landslides, cumulative hydrologic effects of timber removal should be evaluated on the scale of small sub-basins, rather than entire watersheds.