Snohomish Estuary Restoration Effects on Temperature, Salinity, and Tides

From Salish Sea Wiki
Jump to navigation Jump to search

Snohomish River Estuary CWS and CTD monitoring sites.
CWS and CTD locations and the extent of tidally connected wetlands pre-restoration of the Qwuloolt and Smith Island restoration sites in the Snohomish River Estuary.

This study aims to evaluate the short and long-term impacts of estuary restoration to support development recommendations for restoration and resilience planning. This study will be completed in two phases. This page reports the results of Phase I, in which we used data from a system-wide network of continuous water sensors (CWS) and periodic collection of water column profile data (CTD) to evaluate how implementation of two restoration projects in the lower Snohomish River estuary has influenced estuary hydrology, primarily with respect to changes in salinity and temperature patterns. Phase II will explore future conditions in the Snohomish estuary by assessing estuary-wide impacts of large-scale restoration projects on water temperatures, shifts in the mixohaline conditions, and currents by modeling a series of future climate and restoration scenarios. This study is the result of a combined effort by Cramer Fish Sciences in collaboration with the Tulalip Tribes, Salish Sea Modeling Center, NOAA NWFSC, and Snohomish County Surface Water Management Division.


Most large-scale estuary restoration projects involve removal, lowering, or breaching of dikes and levees to restore tidal flooding to lands that had been diked and used for agricultural purposes for decades. The long-term impacts of these land use practices leads to the decay of organic materials and subsidence of estuary habitat. While estuary restoration creates predictable changes to the overall tidal prism, reconnecting tidal influence to subsided sites has been shown to have near-term local impacts on temperature creating warmer and more variable conditions. This is especially pronounced in sites in the lower estuary, where most restoration opportunities tend to be located. Given that elevation and inundation duration are strong drivers of vegetation establishment in estuary systems, these subsided lands may remain unvegetated until tidal, riverine, and sediment supply and transport processes can increase surface elevations sufficient to support vegetation establishment. The period in which recently restored tidal marshes remain unvegetated is of potential concern for restoration effectiveness.

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.

The Snohomish River estuary has been the focus of extensive estuary restoration efforts associated with Chinook salmon recovery plans[1]. To support evaluation of restoration effectiveness associated with individual projects as well as cumulative effects of multiple projects as restoration is implemented in the system, a long-term system-scale monitoring framework was developed. This monitoring framework included deployment of a system-scale network of continuous water sensors beginning in 2010 that has measured salinity, temperature, and water level at 10-minute intervals at key locations through the Snohomish River estuary for over a decade and spans both before and after implementation of two large restoration projects in the lower estuary. These two projects, the Qwuloolt (2015, Ebey Slough) and Smith Island (2018, Union Slough) restoration projects restored tidal flooding to 142 and 124 hectares. Therefore, monitoring data in the Snohomish River estuary provided a unique opportunity to evaluate changes in both local- and system-scale hydrology, and to inform restoration planning in the Snohomish River estuary and beyond.

Study Questions[edit]

  • ·        How does restoring tidal flooding to previously diked areas influence hydrology with respect to key parameters like salinity and temperature?
  • ·        What are the temporal and spatial extent of changes salinity and temperature related to restoration?
  • ·        How are these changes meaningful to juvenile Chinook rearing and restoration planning and evaluation in general?  


As part of the intensive system-wide restoration effectiveness monitoring program, a system-wide network of continuous water sensors and periodic collection of water column profile data were deployed to collect water quality data spanning the periods before and after implementation of the Qwuloolt and Smith Island restoration projects. We used data from these monitoring efforts to evaluate how implementation of these two projects has influenced hydrology; detailed methods will be provided in the final report, but are summarized below.

Specifically we summarized and analyzed the following data:

  • Daily river flow data from water years 1963 to 2022 for the mainstem Snohomish River USGS Station 1215080
    • Classified as extreme low flow, low flow, high flow, or flood conditions[2].
    • Attributed to CWS data.
  • Vertical water column profiles
    • Collected at the mid-channel of distributaries periodically from 2009 to 2022, and more intensively at the Qwuloolt breach site
    • Used to evaluate the maximum extent of salt intrusion and patterns of salinity, pH, turbidity, and dissolved oxygen.
  • Continuous water sensor data
    • Collected water level (pressure), temperature, and salinity (conductivity) at 10-minute intervals at 19 sensor locations from 2010 to 2021.
    • Used to evaluate conditions during peak juvenile Chinook presence in the estuary.

Key Findings[edit]

Available salinity, temperature, and water level data for each CWS site

We found evidence that the Qwuloolt and Smith Island restoration projects reduced the maximum extent of salt intrusion in the estuary (shifting from river kilometer 15.7 to 13.4 in the mainstem distributary) while also increasing the strength of salt intrusion in the lower estuary below the restoration projects.

  • Increases in salinities were strongest in the lower estuary and closest to the projects after each project was implemented (e.g., 0.5-7.1 ppt) and were not confined to the distributary in which the restoration occurred, with implementation of the Smith Island Restoration project resulting in a reduction in salinity near the Qwuloolt Restoration project.
  • We found evidence for significant shifts in temperature patterns, but the magnitudes of these shifts were low at the system and site scale compared to salinity (e.g., 0.3-1.3°C) and the results appear to be confounded by background variation in riverine and marine temperatures.
  • Prior to implementation of the Qwuloolt Restoration project, temperature differences above and below the breach location were relatively similar and we detected a significant increase in temperature differences post restoration when looking at low tides following daytime high tides from February – September. However, we also detected a significant decreasing trend in the temperature differences that we hypothesize could be related to increasing vegetation coverage inside the Qwuloolt over time.

Our results showed that implementation of subsequent projects can result in cumulative changes or additional shifts at system-scales and project impacts are not restricted to the distributaries in which the restoration sites are located. This supports the hypothesis that restoration projects have both a site-level and system-level effect on salinity patterns, and that the effects of restoration are cumulative over time. By calibrating the hydrodynamic models being developed for the Snohomish River estuary (Zackey et al. 2020), we can use these data to ask more direct questions and inform specific restoration scenarios to better understand how implementation of potential restoration projects over time may affect landscape scale salinity patterns.


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

  1. Snohomish Basin Salmon Recovery Forum (SBSRF). 2013. Snohomish River basin 10-year salmon conservation plan and 3-year Work Plan. Snohomish County Department of Public Works, Surface Water Management Division, Everett, Washington.
  2. Indicators of Hydrological Alteration Software Version 7.1