Fish Passage Culvert Program

Video highlights fish passage barriers
The Washington State Association of Counties (WSAC) produced a video highlighting the issue of fish passage barriers and Snohomish County's efforts to address the problem. Watch video to learn more.

Fish Passage Video with Gregg Farris


The Snohomish County Fish Passage Culvert Program is actively working to identify and replace culverts under county roads that act as barriers for fish to swim through and reach upstream habitat.

Snohomish County’s Fish Passage Culvert Program is a coordinated effort between Surface Water Management (SWM) and Public Works. The goals of the program are to:

  • Inventory county-owned culverts to determine fish barrier status. The county is partnering with Washington State Department of Fish and Wildlife (WDFW) to complete inventory by the end of 2021. The WDFW work is funded by a recommendation from the Washington State Association of County Engineers (WSACE)
  • Prioritize fish-barrier culverts for replacement, in collaboration with local tribes
  • Replace barriers with fish passable culverts through SWM standalone culvert projects and Public Works capital improvement projects

Map of County-owned Fish Passage Culverts - open link in new window Opens in new window

View interactive map of county-owned fish passage culverts


What is a Fish Barrier?

Culverts are large pipes beneath roads that allow water to flow, however they do not always provide conditions that allow fish to swim through. A fish passage barrier is anything that hinders fish from moving through a waterway. Originally engineers designed culverts to maximize the capacity to carry water with the smallest possible pipe size. This was efficient and economical. Unfortunately, this design approach can increase velocities, lead to downstream erosion, and perched pipes. The water that flows through culverts may block fish migration because the flow is too swift, too shallow, or has a waterfall into or out of the culvert. A fish-friendly design approach is a structure wide enough to closely mimic the natural channel. Other design elements include the use of slopes and velocities that allow for fish passage, a natural substrate, and length to width ratios that allow the maximum amount of natural light.

Stream Crossing Inventory Database

Snohomish County maintains a Geographic Information Systems (GIS) database of stream crossings throughout the entire county. Currently there are over 4,500 data points that contain bridges and culverts with ownership ranging from federal, tribal, state, county, city, and private. The stream crossing inventory database continuously changes as it is updated with newly collected information. This is the starting point in the county’s process of identifying fish passage projects for the program.

Strategic Approach

Snohomish County follows WDFW's guidelines in the 2019 Fish Passage Inventory, Assessment and Prioritization manual. The process used in the manual to identify if a culvert is a barrier is a time-consuming effort as survey and field data needs to be collected for each culvert. As a result, the county identified the most important sub-basins within the county based on a number of factors including fragmented habitat, water quality, and urbanization. 

A total of eight priority sub-basins were identified within Snohomish County to focus data collection efforts. Once data had been collected in the priority sub-basins the focus turned to the remainder of the county. We are currently partnering with WDFW to finish the inventory of the fish passage culverts within the county. Additionally, Snohomish County shares data back and forth with other resource managers.

Prioritization of Barrier Culverts

Once a culvert has been identified as a barrier it moves into the process of being ranked with other barrier culverts. The first step is to develop a Priority Index (PI) number which is calculated based on a modified version of WDFW’s formula. The WDFW PI equation uses six modifiers for each fish species within the stream reach of interest. The only change to the equation was how the habitat gain was calculated. WDFW manual requests that the habitat gain be calculated by walking the entire stream of interested and measuring stream pools, riffles, etc. to obtain a habitat gain measurement to be used in the equation.

Due to limited staff capacity and funding, Snohomish County cannot obtain habitat information as outlined in the WDFW Prioritization Manual. As a result, Snohomish County developed an alternative approach using in-house resources to develop potential upstream habitat. Although there are some differences between the county’s and WDFW’s approaches on developing priority indexes, the results are similar and will allow ranking of culverts for potential replacement/modification.

Development of a PI as described above is the first step in prioritizing fish barrier culverts. This prioritization list based on PI numbers allows the fish passage team to collaborate with other stakeholders including WDFW and area tribes, to determine which culverts should be moved up or down the list based on other factors that the PI doesn’t account for such as other fish passage work within the basin, natural or man-made barriers, quality of habitat, stream temperatures, water quality, etc.

By the Numbers

As of August 12, 2021, there are a total of 1,731 county-owned culverts:

  • 256 are passable
  • 536 are fish barriers
    • 209 total barriers
    • 327 partial barriers
  • 939 still need to be inventoried and have an unknown status

Successes Since 2015

  • Inventoried 45 percent of county-owned stream-crossing culverts
  • 34 barriers were replaced with new fish-passage culverts
  • Four fish-passage culverts are being constructed in 2021

Increasing Program Efforts

  • 29 projects are currently being designed that will replace 41 fish-barrier culverts over the next three to four years
  • Opportunities to increase the funding and replacement of fish barriers in the coming years are being explored


  • Funding: The cost to replace each fish-barrier culvert typically ranges from $300,000 to $1,000,000. SWM utility fees, REET II revenue, County Road Fund, and grants fund these projects. Fish passage culvert projects are programmed in the SWM Capital Improvement Program (CIP) and the Roads Transportation Improvement Program (TIP).
  • Staffing: SWM and Public Works have been working to increase staff capacity to deliver more high priority fish passage culvert projects. Recruitment of qualified design engineers and project managers has been limited due to various factors (personnel with these skills are in short supply in the PNW region).
  • Permits: Some permits are complicated and unpredictable, and can take as long as 12 months to secure, lengthening project timelines.