FOREST MANAGEMENT AND EFFECTS ON SHALLOW, RAPID LANDSLIDE-INITIATED DEBRIS FLOWS IN OREGON

Shallow, rapidly moving landslides are a dominant erosion process on steep, forested slopes in western Oregon. Most debris flows in Oregon's forestlands are initiated by shallow, rapidly moving landslides. Depending on the geology and the geomorphology, slopes steeper than 65 to 80 percent are most susceptible to debris flow-initiating landslides, especially in the Tyee Core Area. Excavation and drainage changes caused by roads can clearly increase this hazard. Reduction of forest cover by logging, wildfire or other means also increase the likelihood of these debris flow-initiating landslides. The effects on forested slopes that have been clear-cut harvested are most likely related to roots and hillslope water level, and possibly loss of tree canopy. Leaving trees on high landslide hazard locations or reducing the land area with reduced forest cover is expected to limit the increased occurrence of debris flows associated with forest management across the landscape. Stream channel junction angle, gradient and confinement affect the movement of typical debris flows. Multiple initiating landslides, debris left in channels and younger riparian forests may all be factors that increase potential debris flow travel. Large accumulations of wood in debris flow-prone channels may make debris flow behavior less predictable, sometimes increasing the distance debris flows travel. Large trees in the depositional reach of stream channels may decrease the distance debris flows travel. Forest management near debris flow-prone channels differs depending on whether there is a risk to people or to natural resources.