Incipient Motion Criteria Defining Safe Zones for Salmon Spawning Habitat

by Jeffrey B. Bradley, WEST Consultants Inc, Carlsbad, United States,
David T. Williams, WEST Consultants Inc, Carlsbad, United States,
Michael Barclay, WEST Consultants Inc, Carlsbad, United States,

Document Type: Proceeding Paper

Part of: Hydraulic Engineering


Approximately 70% of the City of Seattle's water supply is drawn from the Cedar River. Between 200,000 and 400,000 sockeye salmon spawn annually in this partially dewatered reach. To better determine the allocation of instream flow versus out of stream diversion for water supply, an instream flow study was completed on the Cedar River for the Seattle Water Department. Generally the results showed that optimum habitat was provided at 150 - 175 cfs. Although this flow maximized habitat availability, a concern existed that at this low a flow most of the salmon eggs will be deposited in the center of the stream where they will be exposed to maximum scour potential during winter high flows. The Washington Department of Fisheries preferred that some spawing take place in a 'safe' zone, presumably along the stream margin. In order to define the 'safe' zone an incipient motion model was developed in spread sheet form to evaluate the flow at which salmon redds (eggs are laid in an area approximately 8 - 12 inches deep, and 1 foot in diameter) will be mobilized and the eggs destroyed. Transmitters were buried to depth in the spawning gravel and monitored during winter flows to see at what flow the bed sediments were mobilized. In this manner the incipient motion model could be calibrated and 'risk' and 'safe' zones defined. Instream flows during the spawning season could then be designated to protect the fishery.

Subject Headings: Motion (dynamics) | Rivers and streams | Safety | Streamflow | River flow | Fish and fishery management | Aquatic habitats | Water supply

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