Fluvial connectivity of a deep-seated landslide to upstream tree harvests

Abstract

Channel wall scour may have debuttressed a deep-seated landslide located in a small, mountainous drainage in the Clearwater River Watershed in the western Olympic Mountains, Washington State. Debris flows and high peak flows, both of which can cause channel wall scour, may have been caused by upstream tree harvests. Through examination of stream flow, debris flow and precipitation records relative to the tree harvest and deep-seated landslide activity records, tree harvest effects on the deep-seated landslide are clarified. Following harvest on the landslide, evidence of deep seated landslide activity is undetectable until the trees upstream of the landslide are harvested. In total, three periods of landslide activity are observed. High stream flow (a 25-year event) coincides with one period of activity; however, the magnitude of flow events larger than a 1.1-year event are unaffected by tree harvests. High precipitation and snow melt (water input) events coincide with two of the landslide events but at the time of landslide activity, evapotranspiraiton rates of the plantation trees may have been nearly equal to that of the original forest. In contrast, debris flows, which coincide with all periods of deep-seated landslide activity, dramatically increase following harvest in the headwaters despite below average annual maximum 1-day precipitation and no change in annual maximum 30-day precipitation. Tree harvests in the headwaters of the watershed appear to have caused the increase in debris flow frequency which in turn triggered landslide activity.