FLUVIAL CONNECTIVITY OF A DEEP-SEATED LANDSLIDE TO UPSTREAM TREE HARVESTS

Dendrogeomorphic analysis of a deep-seated landslide that constricts the channel of a small, headwater basin in the Clearwater River Watershed, Washington State, USA, shows that the landslide reactivated at least three times following tree harvests upstream of the landslide. Slope stability analysis of the landslide demonstrates that if pore water pressures are sufficiently high, only one to two meters of scour into the toe of the landslide is needed to debuttress the landslide and trigger activity. Moreover, both debris flows and a large flood (25-year event) occurred at the base of the landslide during periods of activity and may have caused such scour. Results from a lumped hydrology model calibrated to 3 years of flow observations at the basin outlet reveal that the magnitude of flow events larger than a 1.1-year event are unaffected by tree harvests. In contrast, the frequency of debris flows, which may coincide with all periods of deep-seated landslide activity, dramatically increases following harvest in the headwaters despite low annual maximum 1-day precipitation and no significant increase in annual maximum 30-day precipitation. Tree harvests in the headwaters of the watershed appear to have increased the frequency of debris flows which in turn scoured and debuttressed the toe of the landslide.