Fish Passage Culvert Program

Elliott & Connelly Road Culvert, before construction
Elliott & Connelly Road Culvert, after construction

Elliott and Connelly Road Culvert Before Construction

Elliott and Connelly Road Culvert After Construction


The Snohomish County Fish Passage Culvert Program works to restore aquatic habitat by replacing county-owned culverts that prevent fish migration.

The following are objectives for the county's Fish Passage Culvert Program:

  • Develop a single GIS database that contains all culverts within Snohomish County
  • Identify and prioritize county-owned fish barrier culverts
  • Continue collecting culvert inventory in priority subbasins
  • Design and construct projects to provide fish passage at stream crossings
  • Pursue funding for fish barrier removal projects.

What is a Fish Barrier?

Culverts, which are large pipes beneath roadways, may allow water to flow but not provide conditions that fish can 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 increases velocities, leads 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 wide enough for 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 4,000 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 the Washington Department of Fish and Wildlife guidelines in the 2009 Fish Passage Barrier and Surface Water Diversion Screening 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 subbasins within the county based on a number of factors including fragmented habitat, water quality, and urbanization. A

total of eight priority subbasins were identified within Snohomish County and this is where the efforts have been focused. Once data collection and prioritization has been completed for the priority subbasins, the county will continue data collection and culvert assessment on all county culverts. 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 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 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 coordinate with other Surface Water Management (SWM) specialists 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 natural or man-made barriers, quality of habitat, stream temperatures, water quality, etc.

Eight Priority Subbasins

Snohomish County has collected culvert information that allows barrier status determination on county-owned culverts in all eight priority subbasins. The county is currently collecting additional data on culverts in those priority subbasins in order to determine barrier status. 

As of January 2018, there are a total of 396 county-owned culverts:

  • 110 are passable
  • 169 are fish barriers
    • 75 total barriers
    • 94 partial barriers
  • 117 have status unknown and need further analysis
County-owned culverts and bridges overview map.

Fish Passage Culvert Facts (January 2018)

All Stream Crossings

Total stream crossings within county:

  • 308 bridges
  • 4,061 culverts
  • 30 other (dams, fishways, etc.)

Stream crossing ownership:

  • 500 federal
  • 350 state
  • 2 tribal
  • 1,668 county
  • 191 city
  • 1,688 private

County-Owned Stream Crossings

Total county-owned stream crossings:

  • 195 bridges
  • 1,461 culverts
  • 12 other (dams, fishways, etc.)

Total stream crossings with fish distribution upstream:

  • 189 bridges
  • 632 culverts
  • 11 other (dams, fishways, etc.)

Barrier status of culverts with fish distribution upstream:

  • 164 passable
  • 156 partial barriers
  • 75 total barriers
  • 237 unknown barriers