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.

Chronology

2010 - Perimeter dikes at Leque Island breach in high tides and storms.
2012 - Port Susan Restoration by The Nature Conservancy initiates estuary restoraiton, restoring tidal flow to 160 acres of abandoned farmland.
2014 - The deadly Oso Landslide releases a sediment pulse to the delta.
2017 - Zis a Ba Restoration developed by the Stillaguamish Indian Tribe restored 80 acres of Florence Island.
2019 - after a long design process, Leque Island Restoration completes restoration of tidal flow.

Notes

Snohomish Sustainable Lands Strategy has been hosting discussion of how redevelopment of the delta can support both fishery restoration and farm viability.
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.
There is an important role for building community understanding of alternatives and risks in the delta as part of the restoration planning process. Communities living below sea level may have not had the time to absorb and process the implications of sea level rise Pcereghino (talk).
Modelling can fail to resolve social dynamics, because communities can observe detailed system dynamics that are not necessarily observed by simple models. To accurately model delta environments requires an expensive integration of multiple change models. An incremental parallel approach to community development and model development may be useful Pcereghino (talk).

Erosion and Sediment Accretion in the Delta

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.
Sediment is accreting at 3-5 cm/year in the Port Susan Bay Restoration and 1-2 cm/year in more exposed marsh (ESRP pers. comms.)
Marsh continues to erode in northern portions of the delta not receiving river flow. At observed rates fringing marsh along North Florence Island would be gone in 15 years (File:Fuller & McArdle 2014 marsh erosion at stillaguamish delta.pdf). There is currently not ongoing monitoring of marsh recession. The Port Susan Restoration was in part designed on the hypothesis that inadequate sediment delivery from river flow was exacerbating erosion. It is unclear that vertical accretion has an effect on horizontal erosion. An alternate hypothesis could be that in a high energy environment, the delta face only extends under bedload deposition.
Water and sediment from the Stillaguamish River flows predominantly to the south, with wave resuspension assumed as the primary mechanisms for sediment movement to the north.
Fuller completed a brief exploration of Stillaguamish Delta Dredged Sediment Re-use to build marsh in response to marsh erosion.
USGS has assembled a wave energy model for Port Susan Bay, but it is not linked to the 3D flow model developed by Battelle Marine Laboratory

River Delivery of Sediment and Water

Flow is divided between Hat Slough and the Old Stillaguamish Channel. Flow has been switching from the Old Stilly Channel to Hat Slough over time (citation?).
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).
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) affects pore water salinity. In years of low river flow
The drought of 2015 caused a strong reduction in marsh growth due to elevated salinity (TNC pers coms).

Farmland Subsidence, Groundwater and Influence of Sea Level Rise

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 TNC land at the Mouth of Hatt Slough had subsided approximately 1m lower than surrounding marsh prior to restoration.
The Stillaguamish Flood Control District manages (all? some?) dikes and levees in the delta.
Groundwater projection models are based on an extrapolation of limited well sites. Initial work suggests that spring planting could be delayed 5-8 weeks over the next 60 years without pumping.
There is existing saltwater intrusion in parts of Florence Island.

Salmon Recovery

Griffith & Fuller 2012 proposes a specific estuarine restoration target of 80% of historic estuary area to supplement the Stillaguamish Basin Salmon Recovery Plan (SIRC 2005)
There is no current design process for reconnecting the distribution and sediment, freshwater and fish through distributary restoration (as in the Skagit Delta cross island connector.
Leque Island Restoration led by WDFW and Ducks Unlimited, completed internal work in 2017 and anticipated restoration of tidal flooding in 2019 pending state budget resolution.. Initial design elicited concern from some Camano Island residents about how the project might affect their sole source aquifir. A USGS study of groundwater that suggests that the aquifir will not be affected. WDFW is continuing groundwater monitoring during project implementation.
There is a restoration opportunity to improve connectivity at the North Leque Island site, also managed for the state by WDFW.
Port Susan Restoration was completed in 2012 by The Nature Conservancy, initiating restoration of the delta. File:Fuller et al 2014 port susan monitoring report.pdf summarizes findings of monitoring. The project took a "light touch" approach in constructing tidal channels. The natural river levee along the Port Susan Restoration is too high to allow strong freshwater flow into the subsided restoration site. Since restoration, regional studies have suggested that increased breaches would increase connectivity of the site, and improve salmon use. The project included flood gates to reduce flood impact.
The Stillaguamish Chinook stock is critically imperiled, and is one of the Puget Sound "weak stocks" that constrain the North Pacific fishery (KIRO 2019).

Farming

Twin Foods, a major land owner on Florence Island anticipates selling its processing plant in Stanwood [1].
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. Quantification of sea level rise risk through groundwater mounding is important for considering questions about how to protect local agricultural production under greenhouse impacts.

Recreation

WDFW is planning to construct a public boat launch in Stanwood along the Old Channel. The City of Stanwood has recently purchased the old Ovenell Farm for the purpose of developing a multipurpose public waterfront including docks[2].

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.