DESCRIPTION
The final construction for The Nature Conservancy's Port Susan Bay Preserve levee set-back project was completed in 2012, and is categorized as an Estuarine/Nearshore Project with a dike removal. The site is located at the mouth of the Stillaguamish River, on the north side of Hatt Slough, which is currently the main distributary channel for the river. The Stillaguamish Delta is the cornerstone of the Port Susan Bay Ecosystem, and is a contiguous tidal floodplain with the Skagit Delta. The Nature Conservancy's restoration project initiated restoration of the delta, realizing 50% of the 2005 Chinook Recovery Plan's restoration goals for the estuary. The long-term vision for the conservation program was to restore a functional estuarine ecosystem that is resilient to ongoing threats and adaptable to climate change. The primary objectives of this restoration project included: 1) restore self-sustaining native tidal wetlands that support estuarine-dependent species; 2) improve juvenile salmon access to restored rearing habitats; and 3) improve connectivity between the river and northern tidal habitats to promote estuarine function.
Six main actions were associated with this project:
1. Construction included the removal of 7,350 feet of existing sea dike, brought down to the level of the marsh plain.
2. Additionally, two breaches across the levee footprint were excavated to improve connectivity between marsh channels inside and outside the levee. This process re-connected 150 acres of former tidal marsh, which had previously been under agricultural cultivation before field abandonment due to drainage issues. The "starter" channels encourage marsh tidal inundation, drainage, and juvenile salmon access to interior marsh habitat.
3. In order to protect agricultural land adjacent to the site, a new 5,000 foot dike was constructed landward of the restoration site.
4. Community trust and advocacy for the project was developed through a transparent community-involved process in which local landowners were included in the Technical Advisory Committee, and involved throughout the design and construction process. As a result of this process, a $500,000 flood drainage structure was included in construction, which is anticipated to improve flood drainage for Florence Island when upstream dikes are over-topped. Since implementation, there has not been a major flood; the floodgates have thus not been tested yet.
5. 4,200 feet of borrow ditch along the interior of the existing dike was filled in order to better restore hydrology to pre-dike conditions.
6. Post-monitoring work to identify project effectiveness at the site and estuary-scale.
Post-monitoring:
Post-monitoring efforts were supported for a period of 5 years with ESRP funding. The monitoring plan was developed in partnership with USGS, and focuses on the main drivers of the restoration- recovering estuarine ecosystem processes and improving juvenile Chinook habitat. Within each objective, are specific hypotheses and specific predictive or conceptual models. The monitoring plan was developed using a BACI (before-after-control-impact) design, allowing comparison of baseline ecological conditions with those following the restoration, as well as including control (reference) sites and impact (restoration site) comparison to separate restoration effects from regional changes. The restoration monitoring is nested at two scales; the restoration site and the entire estuary system. This approach was chosen because the restoration site is located at the river mouth and will therefore affect system-scale processes. Monitoring elements include landscape, elevation, habitat, hydrology, sediment, and fauna, which together reflect changes in ecosystem structure, function, and processes.
Monitoring results indicate that the estuarine marsh system at Port Susan Bay is not resilient, and is continuing to degrade. The system is likely susceptible to three interacting factors: inadequate freshwater distribution across the delta's face due to levees and dikes upstream, wave erosion, and snow geese soil disruption via foraging activity.
PHOTOS
PSB Dike - photo reference Peter Murray