Stillaguamish Delta

River Delta

River delta sites occur at 16 locations where large river floodplains meet the sea

Most Recent Edits

<DPL> namespace = foo category = river delta ordermethod = lastedit order = descending mode = unordered suppresserrors = true count = 5 </dynamicpagelist>

Salish Sea References

• Ecology Shoreline Photography Viewer
• The Encyclopedia of Puget Sound provides a peer reviewed version of the wiki
• PRISM Project Search
• Washington Coastal Atlas
• UW River History Project

Wiki Rules

• Wiki text does not reflect the policy or opinion of any agency or organization
• Please adhere to our Social Contract and Style Guide
• Complain here, and be nice.

The Stillaguamish River Delta has formed where the Stillaguamish River enters Port Susan, a natural bay formed between the lowland glacial terrace, and Camano Island. The town of Stanwood is partially constructed in the historic delta. Collins & Sheik 2005 consider the Stillaguamish wetlands as part of a contiguous Skagit-Stillaguamish complex.

• Nearshore Portal View of Stillaguamish Delta
• Coastal Atlas View of Stillaguamish Delta

Nearshore Strategies Data Report

Cereghino et al 2012 completed a soundwide analysis to identify and describe river delta sites in Puget Sounds as part of a nearshore ecosystem restoration strategy (using remote sensing data c. 2000-2006). The following narrative of this delta site was developed to support distribution and use of analysis results:

The Stillaguamish Delta in the Whidbey Sub-basin historically contained 7,249 acres of vegetated wetland along a 65 km shoreline. The delta receives flow from a 180,570 square kilometer watershed. These characteristics make this system the 3th largest delta out of 16 systems in Puget Sound.

Simenstad et al 2011 found that this system had lost 69% of its vegetated tidal wetlands, and 22% of its shoreline length. Of the remaining shoreline, 87% shows some evidence of infrastructure development. In the surrounding uplands, 40% of land is estimated to have greater than 10% impervious surface. Across the watershed, 22% of land is estimated to have greater than 10% impervious surface. Based on these paramters, the site was given a degradation score of 40 out of 100, making it the 7th most degraded delta in Puget Sound. It faces a medium risk of future development locally, and a medium risk of development across the watershed. None of the watershed is currently impounded behind dams.

Current Condition and Risks

• The Stillaguamish River is currently not dammed, and so the sediment budget has not been reduced by dam impoundment. However Czuba et al 2011 suggests that the Stillaguamish sediment budget is relatively low compared to other Whidbey Basin systems.
• Flow is divided between Hat Slough and the Old Stillaguamish Channel. Flow switched from Old Stilly Channel to Hat Slough in the 1990s and increased in 2006 flood (Citation?)
• Florence Island and a large area of farmlands behind river levees are below tidal flood elevation, and have subsided significantly since development. Current rates of subsidence are unknown.
• Aerial photo observations indicate that marsh along the face of Florence island is eroding where marsh is not receiving sediment-laden river water.
• The Stillaguamish Flood Control District manages (all? some?) dikes and levees in the delta.
• Along the northern delta, high marsh is accreting but low marsh is eroding along the north. One theory is that wave action combined with snow goose herbivory is eroding low marsh and resuspending sediments. The relative influence of sediment starvation, wave energy, and goose effects are unknown.
• The delta extent described in Simenstad et al 2011 based on River History Project T-Sheets does not extend as far upstream as tidal inundation would currently reach, based on Digital Elevation Models used by the Delta Metrics Project.
• the delta is the cornerstone of the Port Susan Bay Ecosystem
• The delta is frequently considered part of the [[Lower Stillaguamish Floodplain.

Notes

• In 2012 Hatt Slough restoration is complete
• Roger Fuller completed a report for TNC summarizing findings of monitoring.
• Marsh continues to erode in less active portion of the delta. At current rates fringing marsh will be gone in 15 years.
• Water from the mainstem is still predominantly moving out and to the south, and is not bringing accretion to the fringing marsh.
• The natural river berm is too high to allow freshwater flow into the site. Increasing connection of river flow and sediments north of the current river mouth.
• Monitoring suggests that there is accretion in the high marsh zone in areas that are eroding, but the origin of this.
• Biomass production is suppressed in some areas due to pore water salinity. The interaction between topography, sediment texture, river water, and groundwater dynamics (including reduction of groundwater head due to subsidence of historical delta landward), and how these will change under climate change is poorly understood.

Restoration Planning

• Griffith & Fuller 2012 proposes a estuarine restoration target to supllement the recovery plan (SIRC 2005)
• Snohomish Sustainable Lands Strategy has been hosting discussion of how redevelopment of the delta can support both fishery restoration and farm viability, with a focus on the Matterand Restoration and Leque Island Restoration, and an exploration of Stillaguamish Delta Dredged Sediment Re-use to build marsh.
• The Nature Conservancy has completed a dike setback project called the Port Susan Restoration which incorporated flood gates to benefit Florence Island residents and farmers.
• Leque Island Restoration resulted in concern from some Camano Island residents about how the project might affect their sole source aquifir.
• Restoration planning in the system results in concern from farming communities about the loss of farmland.
• Stillaguamish Delta Dredged Sediment Re-use - Snohomish county is exploring the feasibility of how dredge materials could be reused to support marsh formation in the Stillaguamish Delta. Port Susan Restoration is most subsided

Related Wiki Resources

Pages

Documents

particles of clay, silt, sand, gravel, or cobble, transported by water, are called sediment.