Lower Skykomish Floodplain

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Lower Skykomish Floodplain based on old FEMA data

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[edit]

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)

digital elevation model of confluence of Snoqualmie and Skykomish rivers

Landform[edit]

Water[edit]

Biota[edit]

  • 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[edit]

  • 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[edit]

Conservation Efforts (Chronological)[edit]