Snohomish Estuary Restoration Effects on Temperature, Salinity, and Tides

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Snohomish RestorationProjects Zackeyetal.jpg

Estuary restoration projects generally involve reconnection of tidal influence to a previously disconnected site creating predictable changes in overall tidal prism. Most large scale estuary restoration projects restore agricultural lands that have subsided due to the decay of organic materials and land use practices. Additionally, restoration opportunities tend to be limited to sites that are located in the lower estuary. Reconnecting tidal influence to subsided sites has been shown to have near-term, local impacts on temperature creating warmer and more variable conditions. Current restoration strategies assume restoration sites will move toward natural conditions and offer the functional benefits of natural sites after a given amount of time. However, little is known about how restoration actions may change or alter current, and future, conditions within the estuary which may have considerable impacts on expectations of success/effectiveness and influence management decisions for recovery purposes.

  • Previous conceptual models and hypotheses have identified that restoring tidal prism will lead to enhanced water quality conditions including decreasing water temperatures (Puget Sound Nearshore Ecosystem Restoration Project Monitoring Framework).
  • Need to understand how changes to the tidal prism influence local and system-wide metrics and how these compound over multiple restoration projects (cumulative effects).
  • Marsh vegetation in restored areas may be slow to establish and tidal flats without marsh vegetation allow solar radiation to heat shallow waters and exposed ground during low tides and can lead to increased water temperatures
  • Observed at the Qwuloolt restoration site
  • Increased temperatures are exported off the site with ebbing tides
  • Increased water temperatures can reduce DO reduce and subsequent benefits to salmon
  • Understanding cumulative effects of multiple large scale estuary restoration projects on water temperature, salinity, and velocity is essential for better informed restoration, planning, design, and implementation.

Goals and Objectives

Predicting hydrologic changes after estuary restoration has primarily relied on hydrodynamic modeling. This study will use post-restoration data from a hydrographic sensor network in the Snohomish River estuary to quantify hydrological changes after major restoration efforts have been completed. Empirical data to year-long model applications will be compared to validate predictions and use model results to resolve observed changes in temperature, salinity, currents, and inundation. Results can then be used to test scenarios related to restoration potential and response to climate change and SLR projections. The proposed work supports Puget Sound salmon recovery by helping develop recommendations for restoration and resilience planning.

Phase I

Flooding in the Qwuloolt restoration site in the Snohomish Delta

Assess the short-term impacts of large-scale estuary restoration (LSER) projects at site and estuary scales in the Snohomish River estuary by analyzing eight years of water temperature, salinity, and level data, spanning pre- and post-restoration conditions. This analysis will be used to evaluate whether the range/distribution of thermal conditions and/or mixohaline (salinity) habitats change after restoration. This will also help to develop a better understanding of the short-term impacts caused by large-scale restoration and will use fish sampling data to help evaluate the potential impacts on juvenile salmon and inform the design process to ensure these impacts are minimized.

  • Compare of per-restoration and post restoration data in the Qwuloolt and system-wide
  • Compare of upstream and downstream data in the Qwuloolt
  • Compare of data across distributary channels system-wide


  • Report on analysis/assessment of large scale estuary restoration projects effects on site and system-wide water temperature, salinity, and tidal inundation with implication on restoration design.
  • Compiled temperature, salinity, and water elevations for Snohomish Estuary

Phase II

Phase 2 will explore future conditions in the Snohomish estuary by assessing estuary-wide future impacts of large-scale estuary projects on water temperatures, shifts in the mixohaline conditions, and currents by modeling a series of future scenarios, restored and unrestored, under projected future climate change conditions to better understand the estuary-wide and future impacts of large-scale estuary projects on estuary water temperatures, shifts in mixohaline conditions, and velocities that may affect juvenile salmon rearing patterns. This analysis will be used to evaluate cumulative effect scenarios such as (1) how estuarine conditions change with additional (planned/hypothetical) restoration with expected climate change impacts. Phase 2 modeling efforts are likely to change some after results of phase 1 are assessed and from feedback from the advisory group formed under phase 1 of the project. By answering these questions, we will have a better understanding of the potential cumulative short-term impacts of large-scale estuary restoration projects and if these short-term impacts may be exacerbated under future climate change conditions. This information can then be used to further refine estuary restoration designs, strategies, and implementation.

  • Evaluate potential cumulative impacts from full restoration scenario assessed under current conditions
  • Evaluate potential cumulative impacts from full restoration scenario assessed under future climate change scenario.
  • Incorporate updated LiDAR (2019), bathymetry (2020), and data logger network data into new FVCOM model run.


  • Model results showing temperature, salinity, and water elevation system-wide effects for restoration under current climatic conditions
  • Model results showing temperature, salinity, and water elevation system-wide effects for restoration under projected climate change conditions
  • Report summarizing modelling results and implication on restoration design


Please contact Todd Zackey ( with any questions regarding this project.