Smith Island Restoration

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dike removal from top to bottom, with channel re-connections along the way.
This effort is linked to Coordinated Investment pilot work

The Smith Island Restoration Project , led by Snohomish County is planning to restore tidal flow to approximately 326 acres of historical tidal marsh in the Snohomish Delta near the City of Everett. The site is bounded by Union Slough to the east and north, Interstate 5 to the west, and The City of Everett’s wastewater treatment plant and a City of Everett led mitigation and restoration site to the south. Start of construction is planned for 2014.

Funding

Notes

  • First restored tide occurred on August 10, 2018
  • Video of construction at the main breach: https://www.youtube.com/watch?v=eUCqGge_2RI
  • The County has published its Final Environmental Impact Statement, providing the most in-depth analysis of project characteristics.
  • Final design was informed by Tidal Channel Reference Model work by Greg Hood, which indicated that the draft design had many fewer channel openings than a naturally occurring tidal island. Additional connections were included in the final design.
  • The County maintains a website for the project.
  • Like many diked delta sites, most of the delta plain has subsided to low marsh elevations (between Mean Tide and MHWmean high water). Sediment and wood deposition will be necessary to develop marsh or swamp elevations.
  • Very limited data are available to describe sediment dynamics in the Snohomish Delta
  • Some dike retention is planned to reduce costs near high elevation areas, and to protect a buried gas pipeline. This reduces the effective marsh edge, potentially reducing the number of channel entrances below that observed in natural systems. Some hypothesize that number of channel entrances to the marsh plain may affect rearing opportunity (see Effects of delta landscape connectivity on realized function for juvenile salmon)
  • A single large remnant channel will be enhanced to some degree to accelerate tidal channel formation.
  • All previous restoration efforts in the Snohomish has been through much smaller dike breaches.
  • The site is across union slough from the accidental mid-spencer island dike breach, which may provide for a natural experiment in differential connectivity.
  • A portion of the site, south of the gas pipeline, is being developed for mitigation credit.

Near infrared aerial image of project site in 2011

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