Study shows fecal bacteria levels have declined in the Snohomish River Basin

Over the course of many decades, human activities led to water polluted by fecal bacteria in the Snohomish River Basin. Today, the region is seeing reduced bacteria levels and cleaner water because of projects stewarded by local partners. The projects were informed by previous environmental studies and a water cleanup plan.

We conducted an effectiveness monitoring study to measure the impact of clean water projects in the Snohomish River and its tributaries. The study details where bacteria levels have improved and what areas face pollution challenges.

Environmental scientist Sarah Yepez, who leads the basin's watershed cleanup process, said effectiveness monitoring studies show us where cleanup efforts are working and where we need to focus efforts to reduce pollution.

"The Lower Snohomish effectiveness monitoring study helps us prioritize clean water projects and guide local action to reduce bacteria pollution," Yepez said. "We are working with partners in Snohomish County to keep up the momentum, so everyone has access to clean water."

Sustained efforts to reduce pollution

The Snohomish River Basin, which is the second-largest river basin in Puget Sound, has changed rapidly since the mid-1800s as more people migrated to the area. Increasing populations of people and livestock led to a significant increase in fecal bacteria levels.

Fecal bacteria are found in human and animal feces and naturally exist in the environment. But when we find high concentrations of fecal bacteria in rivers, streams and lakes, it often means that people and animals can be exposed to diseases. This puts anyone swimming, playing, fishing, or boating at risk for illness.

Previous studies by the Tulalip Tribes, the state, and Snohomish County detected high levels of fecal bacteria in many locations from many sources, including agriculture, septic systems, wastewater treatment failure, stormwater runoff, and lack of filtration due to deforestation.

In response, we prepared a water cleanup plan in 2001 that set targets to limit bacteria pollution and identified local actions to improve water quality.

Since then, Tribal, state, and local governments and other partners have made a concerted effort to work with communities to manage livestock manure, repair septic systems, reduce stormwater pollution, upgrade wastewater treatment plants, restore river and stream habitats, and expand community education and outreach programs.

Impact of local actions

Are local efforts working? To answer this question, environmental scientist Niamh O'Rourke collected and analyzed water samples in 2020 and 2021 from 46 locations - 36 locations where previous studies had reported high bacteria levels and 10 investigative sites to trace where pollution may be coming from.

Study results, published in 2025, showed that in most sampled areas, bacteria levels have declined or remained steady despite significant population growth and increase in land development. However, some areas still show high or worsening bacteria concentrations, particularly Allen Creek and Quilceda Creek. This could be due to dense clusters of aging septic systems in those locations.

O'Rourke said a few sites had low enough bacteria levels to be considered for a status change on the state's list of polluted water bodies (also called the 303d list). In the last update of the list, the status of two water bodies - Pilchuck River at Menzel Lake Road and a tributary of French Creek - has changed from polluted to clean. One site in Quilceda Creek meets water quality standards for fecal bacteria. This means three more sites in the basin have improved, and we can focus cleanup efforts on waterbodies that need more resources.

"Water quality has improved in many tributaries, reinforcing that we're on the right path to making these rivers and streams safe for recreation," O'Rourke said. "We appreciate community members and our partners for their continued work to reduce pollution in the Snohomish River Basin. Every effort adds up over time to make a big impact."

Get involved

Everyone can collectively make a difference in their community and help protect and cleanup local waters by:

• Maintaining your septic system: Take care of your septic systems to avoid bigger or more expensive repairs. For septic system grants or rebates, visit the Snohomish County's Savvy Septic Program website. If your septic system needs repairs or replacement, Ecology partners with nonprofit lender Craft3 to offer affordable clean water loans.
• Controlling runoff: Reduce pollution from your property that ends up in rivers and streams. For free services to help control rain runoff on your property, reach out to the Snohomish Conservation District.
• Taking actions for clean water: Pick up pet waste, report spills, plant a rain garden or practice natural yard care.

As we continue to address water quality challenges in the Snohomish River Basin, it is encouraging to see that local research, conservation and policy actions are helping restore the river and its tributaries.