Fluvial Geomorphic Adjustments Following a Channel-Damming Landslide in the Cedar River, Washington

Following more than a century of anthropogenic modifications to the Cedar River, Washington, a channel-damming landslide changed the fluvial landscape when approximately 50,000 m³ of sediment was deposited in the channel during an earthquake. Emergency crews removed bank hardening and redirected flow into an existing floodplain side channel.  We measured the channel for three years following the disturbance and compared that to one year of pre-disturbance data for approximately 1 kilometer of channel – immediately upstream, in the directly-impacted reach, and adjoining downstream reach. In the directly impacted reach, rapid lateral adjustments recruited large trees to the channel and dramatically increased physical habitat heterogeneity. Channel widths increased 250%, river bed sediment gradation nearly doubled from 2 to 4, and large wood loads approached 150 pieces ha^-1. Channel conveyance increased from dramatically during the study period in the landslide reach but decreased downstream as sediment loads were transported downstream. This had the effect of increasing channel and floodplain interactions downstream of the landslide impact area.