South Fork Necanicum River Fish Passage & Screening Enhancements


HENDERSON design-build professionals assisted with the Necanicum Watershed Council in the removal of an existing diversion dam, relocating and reconstructing the City of Seaside’s screened municipal water supply diversion on the Necanicum River. The mainstem and South Fork Necanicum provide spawning and rearing habitat for Coho, winter steelhead, fall Chinook, coastal cutthroat, and chum salmon. Though the City’s channel-spanning concrete diversion dam has an antiquated fish ladder, it is a fish passage barrier for juvenile fish and a partial fish passage barrier to adults depending on seasonal flows and velocities. This dam also impedes movement of coarse sediment through the system, resulting in the sequestration of alluvial material behind the dam negatively affecting downstream habitat through disruption of bedload transport. This lack of seasonal transport results in downstream habitat scoured to bedrock in most years. Further, water withdrawals to supply the city water system render the South Fork Necanicum dewatered during low flow months in some years, impacting miles of downstream habitat and negatively impacting stream temperatures.

HENDERSON professionals worked closely wi th representatives from the City of Seaside, NWC, USFWS, and ODFW. Challenging project objectives included providing for unimpeded upstream and downstream fish passage for all life history forms of all historically present species in a manner which meets state and Federal regulatory requirements while relocating and diverting gravity-fed municipal flows to the City of Seaside throughout construction. HENDERSON field crews removed the existing low-head water diversion dam and pump house, constructing a stable engineered boulder and cobble stream channel to re-establish fish passage with sediment and large wood transport through this reach.

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