ANTHROPOGENIC GEOMORPHIC SIGNATURES OVERPRINTED ON THE NATURAL HILLSIDE GEOMORPHOLOGY AND HOW IT RELATES TO LANDSLIDE INTERPRETATION

Geomorphology is an essential part of the landslide mitigation studies in engineering geology; especially with respect to timber harvest plans (THP). The bedrock geology of northwestern coastal California is highly variable with regard to rock strength, lithology, and structural fabric. Landforms developed in a temporal climate in conjunction with rapid regional uplift and strong ground shaking driven by the Cascadia subduction zone and the northward migration of the San Andreas fault. This results in highly dynamic and varied topography. The topography is then overprinted by shallow mass wasting processes and deep-seated landsliding and further modified by anthropogenically imparted expressions.

We have learned that prior knowledge (history) of the land surface activities results in a more refined interpretation of the geologic processes. Turn of the century, highly disruptive, timber harvesting has resulted in present day identifiable geomorphology. Geomorphic evidence of historical timber harvesting observable today includes: deep 'V-notched' yarding furrows; steam donkey trails; and uprooted old growth stumps. Further evidence to support anthropogenic geomorphology is observed in the remaining timber that was not taken to the mill and still exists in the forest. This includes, saw-cut old growth stumps, downslope trending of felled timber and stacks of firewood for steam donkey use. Natural drainage systems were often disrupted and concentrated to historical yarding corridors creating streams. Separating the anthropogenic modifications from the underlying geology yields more refined interpretations with regard to hillslope response. With age constraints of the initial timber harvesting, we can observe the development and relative rates of numerous hillslope processes. These processes include the erosive tendencies of the soils and the fluvial incision within corridors, unconsolidated fill settlement and the weathering rates of varying textured soils and organic fills. In a sense, the morphology observed within the harvest unit present day is two fold: it is the result of natural geologic processes modified by anthropogenic input. Where landsliding has occurred, we can assess, forensically, the inputs from anthropogenic activities and mitigate the methods utilized in the next proposed harvest.