Lower Snohomish River Tributaries effectiveness monitoring

High levels of bacteria like fecal coliform and E. coli indicate the presence of human or animal poop in tributaries of the Lower Snohomish River. Fecal pollution can expose people to pathogens that pose risks to human health. Sources of fecal pollution can also cause other water quality issues that threaten fish and fish habitat.

Monitoring and historical trends

We collected water samples at 50 sites across these six watersheds in the Lower Snohomish River basin in 2020 – 2021: 

• Allen Creek
• Quilceda Creek
• Woods Creek
• French Creek
• Pilchuck River
• The Marshlands

We analyzed water samples for fecal coliform bacteria and E. coli and determined that most of the revisited sites had lower fecal coliform levels than in the initial studies from the 1990s. Most of our study sites still had fecal coliform and E. coli results that exceeded state water quality standards. We also analyzed historical data for trends and found three sites with decreasing fecal coliform levels and four sites with no trend detected.

Study area assessments and conclusions

In addition to studying water quality, we evaluated how much cleanup work had been done in the watershed and assessed other study area conditions, including land cover and population changes and septic system density. We compared these assessments with water quality results and determined that the significant cleanup work that has been completed likely contributed to the declines we saw in fecal pollution in the study area.

However, more work is needed. There are a variety of potential sources to address, including septic systems in the Allen and Quilceda Creek watersheds that may be contributing pollution and dampening the success of other local cleanup efforts in those areas.

View results

Our 2025 final study report, Lower Snohomish River Tributaries Fecal Coliform Bacteria Effectiveness Monitoring, describes our assessment results and recommendations.

An interactive story map: Bacterial Pollution in the Lower Snohomish Basin was published in 2021 and includes maps, data visualizations, and preliminary results from the study.

See below for links to the full study data and other related publications.

Publications and data

• Study data (2020 – 2021)
• Lower Snohomish River Tributaries Fecal Coliform Bacteria Effectiveness Monitoring Final report (2025)
• Quality Assurance Project Plan: Lower Snohomish River Tributaries Fecal Coliform Bacteria Effectiveness Monitoring (2021)
• Lower Snohomish River Tributaries Fecal Coliform Bacteria TMDL: Detailed Implementation Plan (2003)
• Snohomish River Tributaries Fecal Coliform TMDL Submittal Report (2000)
• Water Quality Assessment of Tributaries to the Snohomish River and Nonpoint Source Pollution TMDL (1997)