Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7519
Title: Characterization of annual salinity and temperature patterns in a large river delta to support tidal wetland habitat restoration efforts
Author: Jason E. Hall, Tarang P. Khangaonkar, Casey A. Rice, Joshua W. Chamberlin, Todd Zackey, F. E. Leonetti, Michael Rustay, Kurt L. Fresh, Anna N. Kagley, M. Rowse
Publication Year: 2018
Journal: Northwest Science
Volume: 92
Issue: 1
Pages: 36-52

Although the Snohomish River estuary remains the second largest tidal wetland complex in Puget Sound, approximately 90% of pre-settlement habitat has been disconnected from tidal exchange.  This estuary is currently the focus of the largest restoration effort in Puget Sound, with opportunity to restore tidal exchange to over 50% of pre-settlement levels.  The Snohomish River also currently supports populations of all anadromous Pacific salmon species, including Endangered Species Act listed Chinook (Oncorhynchus tshawytscha), steelhead (O. mykiss), and bull trout (Salvelinus confluentus).  The combination of extant anadromous Pacific salmon populations, large existing tidal wetland complexes, and large restoration potential make the Snohomish River estuary a great opportunity to benefit salmon population recovery and conservation efforts.  To support restoration planning and effectiveness monitoring, we developed baseline characterizations of key physical attributes (salinity and temperature).  Our results indicated that brackish (0.5-30 ppt) conditions extended farther upriver than previously described, with distributary channels downstream of the middle mainstem and lower Ebey Slough remaining brackish throughout most of the year.  During extreme low flows (< 0.65 m3 s-1), salt water (> 0.5 ppt) can at times intrude throughout the distributaries and up to river kilometer 15.9 above the first bifurcation.  We also observed temperatures exceeding stress thresholds for juvenile salmonids throughout the estuary from July through September, a period that overlaps with juvenile rearing.  This research is timely with several large restoration projects scheduled for construction by 2020, and these baseline characterizations can be used to evaluate restoration responses, as well as to inform project prioritization and monitoring.


 This report details the results of an investigation into the mixohaline habitat zones of the Snohomish River estuary. 

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Theme: Habitats to Support Sustainable Fisheries and Recovered Populations
Foci: Develop effective and efficient habitat restoration and conservation techniques.
Characterize the interaction of human use and habitat distribution, quantity and quality.