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.

(From SBSRF 2005)

From Sultan to Monroe, sedimentparticles of clay, silt, sand, gravel, or cobble, transported by water, are called 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, sedimentparticles of clay, silt, sand, gravel, or cobble, transported by water, are called sediment. deposition increases and channel stability decreases [approaching the confluence], but conditions, land uses, and restoration opportunities are similar.

(From NHC 2006)

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.

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.

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.

digital elevation model of confluence of Snoqualmie and Skykomish rivers

Notes on Conditions

  • 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
  • DeVries 2010 provides a geomorphic assessment of the Lower Skykomish floodplain
  • NHC 2006 describes flood risk as part of FEMA assessment.
  • USGS Gauge near Gold Bar - continuous flood level readings.

Reach Scale Planning

Snohomish County has been developing reach scale plan methods with Skykomish as a pilot.

Notes of Activities