French Slough Floodplain and Watershed

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map of floodplain and surrounding lowland watersheds

The French Creek watershed drains 28 square miles in south central Snohomish County and is about seven miles long and six miles wide. French Creek empties into the Snohomish River just south of the City of Snohomish at river mile 14.3. Approximately 12 percent of the watershed is in City of Monroe; 88 percent is in unincorporated Snohomish County.

The watershed contains about 115 miles of streams and ditches and supports populations of coho salmon, searun cutthroat trout, and steelhead trout. The French Creek floodplain once included a 4000-acre scrub-shrub wetland in the Snohomish River floodplain. (FCWMC 2004).

Agricultural use is enabled by a pump station and drainage channel network, managed by the French Slough Flood Control District. The pump station has a fish ladder which does not allow for full fish passage, located where French Slough meets the Snohomish River. In the slough, summer temperatures are high enough to injure fish, and results in low oxygen levels. The site is listed (303d) for Clean Water Act violations. Suburban and agricultural land use contributes nutrients that exacerbate low oxygen levels. Rural residential development is increasing in the uplands, between the towns of Monroe and Snohomish, increasing runoff.

Ongoing Work[edit]

This effort is linked to Coordinated Investment pilot work


Ditch network in French Creek floodplain
  • The French Creek levee is designed to overtop in a controlled manner during a 5-year flood. The French Creek Levee has been damaged by Pilchuck flooding (need citation).
  • There are over 240 farms in the watershed. Three dairies have from 800-1000 animals each. 66% of non-dairy farms raise horses, 20% cattle, and 8% other grazers. Fecal coliform standards are exceeded throughout the watershed.
  • The county maintains a roadside ditch system that is regulated under a Snohomish County NPDES permit.
  • The Tulalip Tribes re-typed streams in 1997 to correct errors, but new typing is not used by DNR, City of Monroe, or Snohomish County as of 2004.
  • WDOE is conducting a TMDL for French Creek and the Pilchuck River focused on temperature and dissolved oxygen. Initial data collection is underway.
  • Extensive hydrologic modelling of zoning build-out was completed in 1997 and is summarized by FCWMC 2004.
  • File:FCWMC 2004 french creek watershed management plan.pdf - the result of a county led planning process, endorsed by Ecology, come to the following conclusions:
    • Current development planning (2004) plans for higher development in the more sensitive headwaters. Build-out is anticipated to result in loss of all but 1100 acres of forest over 28 square miles, and 14.7 total impervious area, potentially destroying salmon runs. Current levels are roughly 33% forest cover, and 6.6% impervious (p. 77).
    • Hobby and horse farms are impairing water quality. Water quality in the floodplain is lethal to salmon.
    • Shrubs and grasses dominate 50% of buffers and less than 15% of streams have buffers greater than 100 feet.
    • 66% of remaining wetlands are less than one acre in size, making them very vulnerable to further degradation.
    • Over the last year there has been a 1 foot drop in the underlying Lakes aquifer for unknown reasons.
    • The report recommends: 1) reduce zoning from Rural 5 Basic, to Rural 5, 2) improve clearing and grading ordinances, 3) build a French Creek tidegate at the Snohomish River., 4) maintain wetlands and buffers, 5) maintain water facilities, decrease impervious and protect forested areas. 6) enforce regulations with fines rather than just 'technical assistance and referral' approach.
    • Watershed residents responded to a survey asking which watershed attributes were essential to maintain. They wanted to preserve their quality of life and the fish and wildlife habitat that are important to them, through maintaining forests, wetlands, and riparian areas. These are the same features that biologists have found would maintain a healthy stream system and watershed. Determining how to reverse the degradation trend was a challenge for the WMC, since an assessment of future land use indicates that all but 11 percent of the watershed’s forests will be removed. Wetlands are inevitably mitigated, and studies conducted by King County have found that almost all wetland mitigation projects fail. Riparian buffers continually are diminished; even regulated buffers do not provide full function for all species.
  • WDFW 2015 describes a 5-year HPA permit for drainage ditch maintenance.
  • SEE 2015 was developed to support implementation of that permit, and describes the ditch structure in the floodplain.

French creek watershed.jpg