Lower Skykomish Floodplain

Floodplain

Floodplains occur where large rivers and streams meander in their own sediments

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

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The Lower Skykomish Floodplain extends from the confluence with the Sultan River at the City of Sultan, downstream to the confluence with the Snoqualmie in the vicinity of the City of Monroe. Major tributaries to this reach include the Sultan River Watershed from the north at Sultan, the Elwell Creek Watershed from the south, and the Woods Creek Watershed from the north at Monroe. Downstream of Haskell Slough the basin is known as Tualco Valley.

Document Excerpts

From Sultan to Monroe, sediment supply and deposition balance out, and the channel becomes naturally more stable. It continues to support substantial Chinook salmon spawning and rearing [compared to the braided reach upstream], but is squeezed by a high amount of bank armoring that isolates the mainstem from off-channel habitats and prevents channel migration. As in much of the basin, current large woody debris loading and riparian conditions are a small fraction of their historic levels. Downstream of Monroe, sediment deposition increases and channel stability decreases [approaching the confluence], but conditions, land uses, and restoration opportunities are similar. (SBSRF 2005)

Flood problems in the lower Skykomish River are concentrated in the Tualco Valley area south of Monroe. The highest concentrations of development in the City of Monroe are generally on high ground above flood levels. Skykomish floodwaters do, however, back up Woods Creek and also inundate the Cadman gravel pit, both within the Monroe city limits.(NHC 2006)

The principal infrastructure affected by flooding includes SR-203, which stretches south from Monroe across the Skykomish River and through the lowermost portion of the Snoqualmie Valley, and the Burlington Northern - Santa Fe (BNSF) railroad, which parallels the river and is near the main channel at several points where it must be protected from bank erosion. Haskell Slough, a major floodplain overflow path, is blocked at its head by the Haskell Slough dike, a Snohomish County maintained structure. This dike, and the lands adjacent, has sustained significant damages in past floods and the river has exhibited behaviors indicating a possibility of avulsion though the slough.(NHC 2006)

In the lower end of the Tualco Valley, the flows of the Snoqualmie and Skykomish Rivers merge. Flood flows here are slower, but depths of flooding are greater and durations are longer. Throughout the lower Skykomish Valley localized bank erosion is common, putting structures near the channel at risk.(NHC 2006)

Landform

• DeVries 2010 provides a geomorphic assessment of the Lower Skykomish floodplain
• NHC 2006 describes flood risk as part of FEMA assessment.
• File:Cardno 2020 lower skykomish channel migration zone.pdf delineates the area in which channel migration may occur, which is part of the state and federal Regulation and Mitigation Systems
• File:Cardno 2021 lower skykomish geomorphic assessment.pdf provides more detailed analysis of locations where the river is most likely to change in the near future including areas where that change could be used to increase fish habitat, and where that change will create flood hazards. Maps from appendices are uploaded as a separate document: File:Booth et al 2021 lower skykomish geomorphic assessment appendices.pdf

Water

• USGS Gauge near Gold Bar - continuous flood level readings.
• The Haskell Slough Levee may limit flushing flows into Riley Slough (Haring 2002).
• Garland & Liszak 1995 describes the High Rock Aquifir Break, where a gravel mining operation broke through a perched aquifer, causing a decline in 13 wells and three natural springs that had historically provided irrigation water, dairy water, and streamflow for the Tualco Valley.
• Snohomish County has completed extensive Flood Hazard Mapping as part of their Community Floodplains Solutions effort.
• File:Mauger et al 2015 snohomish downscale hydrologic projections.pdf - provides predicted future river flows under climate change used to inform hydraulic modeling.
• File:WSE 2021 snohomish hydrologic and hydraulic modelling.pdf provides evaluation of flood depth and velocity for 10-year flood events, including mid and late-century estimates of expanded flood area.

Biota

• SBSRF 2005 suggests that Skykomish Chinook are at 3.4% of historic equilibrium population. The lower Skykomish Floodplain has among the highest current levels of Chinook use in the Skykomish system.
• WDFW completes Chinook spawner surveys to estimate escapement with data provided at theSCoRE. Additional information is avaialable about Snohomish system stocks
• No coho spawners have been observed in the Riley Slough/Foye Creek system since 1994.
• There is a large beaver dams complex at the mouth of Riley Slough that should have coho rearing, but where coho rearing has been conpicuously absent (Haring 2002).

Society

• 20% of the population in areas of the City of Monroe speak Spanish at home.
• The Werkhoven family dairy enterprises are large lease holders and cultivate forage for their dairy operations in the valley.

Land Use

• This is an assessment reach in the Snohomish Sustainable Lands StrategySnohomish County has been developing reach scale plan methods with Skykomish as a pilot.
• https://snohomishcountywa.gov/3808/42545/Reach-Scale-Plans - reach scale plan with Lower Skykomish Pilot
• Lower Skykomish Floodplain Land Strategy is a project to organize county acquisition of parcels important for Integrated Floodplain Management
• Expansion of the Cadman, Inc. mining operation prompted development of People for the Preservation of Tualco Valley which does not appear to be active (2022)
• File:Cardno 2021 skykomish river infrastructure investigation.pdf describes the condition of existing river levees.

Conservation Efforts (Chronological)

• 1997-2003 - The Haskell Slough Restoration project was an early action on the Reiner Farm that aimed to increase flows to the slough, underplant conifers in riparian forests and use flood fencing to reduce avulsion risk and coarse sediment deposition on floodplain farm land.
• 1999 - Northwest Chinook Recovery first explores restoration of Groenveld Slough (RCO contract 99-1302) but it appears no work was completed.
• 1999-2002 - The Snohomish Conservation District and Stilly-Snohomish Salmon Enhancement Task Force begin revegetation around Riley Slough (RCO contract 99-1401)
• 2001 - Six farmers owning 1500 acres downstream of Sultan form the Lower Skykomish River Habitat Conservation Group, develop an HCP leading to the Groenveld Slough Project. (from this Shared Strategy profile)
• 2002-2007 - - Snohomish County completes the Skykomish Braided Reach Project experimenting with flood fencing to guide river evolution.
• 2005 - Construction completed on a biodigester forming the Qualco Energy Corporation to turn cow manure into energy and biomass, as a shared venture between the Werkhoven Dairy, Tulalip Tribes, the Sno/Sky Agricultural Alliance, and Northwest Chinook Recovery, as documented in an article in Biomass Magazine
• 2005 - The Skykomish is listed by American Rivers as one of "America's Most Endangered Rivers"
• 2007 - Skykomish Habitat Mitigation Bank releases credits from a private wetland bank authorized by WDOE.
• 2008-2010 - Sound Salmon Solutions completed assessment, wood placement and reforestation along upper Tychman Slough (RCO contracts 08-1578 and 10-1186), the future site of the Mann Road project.
• 2010-2011 Snohomish County works with landowner along Groenveld Slough on river right between RM 10-13 (sometimes now referred to as the Sultan Reach) to install flood fencing and reforest (RCO contract 10-1338 and 11-1238). This area and some installations are later damaged by floods.
• 2012 - Snohomish County works on conceptual designs for adding ELJs and woody revetments upstream of the Snoqualmie/Skykomish confluence (documented in three PRISM contracts: (10-1338, 12-1251, and 11-1238) but these were never constructed.
• 2016-date - Adopt-a-stream Foundation begins planning and design for the removal of the Woods Creek railroad bridge crossing through Al Borlin Park (See RCO contracts 20-1135 and 16-1639
• 2016 - the Washington Farmland Trust is funded to acquire and perpetually protect the Reiner Farm including a collaboration with the Tulalip Tribes to protect and restore riparian forests along the river frontage. (with an RCO contract for both Farmland protection] and riparian protection.
• 2018 - Snohomish CD funded to replace Carl Peterson Fish Barrier (RCO contract 18-2028)
• 2019 - A $5M award from the Floodplains by Design program supports large scale assessment of the Lower Skykomish.
• 2020 - Tulalip Tribes initiates its Snohomish River Restoration strategy, prioritizing parcels with the highest value for salmon recovery RCO contract 20-1139.
• 2020 - Tulalip Tribes are funded to explore strategies for increasing river flow in Haskell Slough to restore salmonid rearing habitat. (RCO Contract).
• 2021 - Snohomish County wins a second $8.8M Floodplains by Design award, accelerating feasibility and design work and formally initiating the Community Floodplains Solution program.
• 2022 - Snohomish County releases a series of analyses describing climate change impacts of flood elevations, the condition of river infrastructure, a delineation of the channel migration zone, and assessment of habitat opportunities and flood hazard risks along the Lower Skykomish, setting the stage for large scale river restoration.

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