The Role of Forest Harvest Practices and Extreme Events on the Sedimentation of an Oregon Coast Range Lake

Timber harvesting and road building in steep mountainous landscapes alter hydrologic routing and sediment delivery to streams. Fires and earthquakes destabilize hillslopes. However, how closely coupled these disturbances are to extreme precipitation throughout an entire catchment is not well understood. We use lake sediment as a record of catchment events through time. By analysis of sediment cores, we quantify historical changes in sedimentation after large-scale timber harvest as well as after implementation of BMPs in contemporary forest harvest. Measuring sedimentation rate and characterizing sediment by particle size, %C, C/N, and magnetic susceptibility, we show changes in the system in response to disturbances. Applying the hydroclimate record, we analyze the impact of extreme precipitation events and their interaction with disturbance. We also consider changes during and after earthquakes, of which four have likely occurred during the lifetime of this landslide-dammed lake. We examine the stratigraphic fidelity of lake sediments as a record of catchment events. Preliminary results suggest that mean sedimentation decreased after Oregon Forest Practices Act. Sedimentological changes offer a glimpse over a 1500+ year time period of fluvial and lacustrine processes of a single catchment, and how the system may respond to increasing precipitation intensity under different management regimes.