Stillaguamish Delta

River Delta

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

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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

Context

• The Stillaguamish Delta is the cornerstone of the Port Susan Bay Ecosystem, and is sometimes geomorphically grouped with the Skagit Delta (Collins & Sheik 2005) as a contiguous tidal floodplain system with two rivers. In the farming community, the delta is frequently considered part of the [[Lower Stillaguamish Floodplain. 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 modern Digital Elevation Models reviewed by the Delta Metrics Project.
• 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.
• 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.
• The Oso Landslide in March of 2014 was a gigantic slump in a deep seated landslide site on the North Fork Stillaguamish Floodplain, and increased sediment loading by 50 or 60% for several years, after a large initial pulse. Much of the Oso material has largely moved through the system, due to the high proportion of fines in the slide material resulting is a large proportion of suspended load (Grossman, pers coms).
• 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?).
• The Stillaguamish Flood Control District manages (all? some?) dikes and levees in the delta.
• Twin Foods, a major land owner on Florence Island anticipates selling its processing plant in Stanwood [1].

Related Efforts

• Griffith & Fuller 2012 proposes a estuarine restoration target to supplement 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 the Matterand Restoration and Leque Island Restoration.
• Fuller completed a brief 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 generates 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

Restoration

• In 2012 the Port Susan Restoration was completed by The Nature Conservancy, initiating restoration of the delta. File:Fuller et al 2014 port susan monitoring report.pdf summarizes findings of monitoring.
• Marsh continues to erode in portion of the delta not receiving river flow. At current rates fringing marsh along North Florence Island would be gone in 15 years.
• Water from the Stillaguamish River mainstem is still predominantly to the south.
• The natural river levee along the Port Susan Restoration is too high to allow strong freshwater flow into the subsided restoration site. Increasing connection of river flow and sediments north of the current river mouth has been proposed.
• Along the northern delta, high marsh is accreting but marsh area is still eroding. The origin of high marsh sediment inputs is unclear. One theory is that wave action combined with snow goose herbivory is eroding low marsh and re-suspending sediments. The relative influence of sediment starvation, wave energy, and goose effects are unclear.
• Primary production in tidal marsh is affected by 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. Years of low river flow, and accompanying increases in low-flow salinity can change marsh vegetation episodically. Vegetation provides wave attenuation.
• Ongoing work at Leque Island Restoration and Zis a Ba Restoration
• What are our expectations for the role of restoration in affecting freshwater conveyance into the system, and what is our strategy relative to subsided areas within the delta that are not typical
• Snohomish Agricultural Resilience work is trying to consider the combination of groundwater mounding, increased river level due to both bed aggredation and increased hydrograph. Proposed groundwater analysis will evaluate groundwater work for both the Stillaguamish and Snohomish.
• This raises questions about how to manage agricultural land during the complex transition.
• It seems likely that the re-routing of the Stillaguamish River mouth at TNC may not be significant enough to cause system scale effects, however breaching the TNC site to increase river flow into the restoration site.
• There is an important role for building community understanding of alternatives and risks. Modelling can fail to resolve social dynamics, because communities understand the dynamics of the system that are not resolved by monitoring.

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.

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