Client: Columbia Soil and Water Conservation District
The Lower Clatskanie Floodplain Restoration Project site is located along the mainstem of the Clatskanie River approximately 2 river miles upstream of the Highway 30 crossing near the City of Clatskanie. The project reach is located where the river transitions from a riverine to a tidal regime, the floodplain widens considerably, and coarse bedload deposition is greatly enhanced. Consequently, the river is very dynamic and the floodplain consists of a mosaic of secondary and tertiary channels that are a result of historic and recent avulsion events associated with high flow, localized sediment deposition, bank failures and landslides, and development of large log jams. Although the site appears to be under the influence of natural physical processes, past land use impacts have led to a degraded riparian corridor dominated by reed canary grass, less frequent side channel connectivity, and a lack of pool scour in floodplain and secondary channels.
To address these impacts, Waterways Consulting was hired to assist the Lower Columbia Watershed Council and the Columbia SWCD to evaluate restoration opportunities and constraints, prepare a set of restoration alternatives, estimate construction costs of the proposed alternatives, and prepare a preliminary engineering design for the selected alternative. The selected alternative focused on moderate grading in key locations of the floodplain to enhance side channel connectivity to the mainstem, installation of localized log jams to enhance scour in the target side channels, and placement of large wood on the floodplain. These efforts will be combined with a comprehensive revegetation effort and targeted control of reed canary grass.