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Fish Passage Solutions

Humboldt CalPoly
Fish Passage Engineering Project - Site Analysis Final Report

In 2019, Caltrans Division of Research Innovation and Systems Information (DRISI) initiated the Fish Passage Engineering Project with the objective of evaluating the implementation of recent fish passage barrier remediation at State Highway System (SHS) road crossings.


Crossings that span the bankfull channel width were of primary interest due to Caltrans’ Fish Passage Priority Program and FishPAC goals of maintaining geomorphic continuity, providing connectivity for fish and wildlife.  

Download the Full July 2022 Report with Appendices here.

Humbolt CalPoly Engineering ReportCover.png

Click on the document to download the full Fish Passage Engineering Project - Site Analysis Final Report.


Stream Passage Solution Priorities

When evaluating a potential road/stream crossing design and solution that prioritizes fish passage for habitat enhancement and improved migration for aquatic and terrestrial species, the following alternatives and types of structures should be considered. The priority order of solutions considered results in the maximum benefits to terrestrial and aquatic species, and the minimum long-term costs to the agency. Enhancing wildlife connectivity for all species helps native fish and wildlife more resilient to the effects of climate change stressors, such as increased stream temperatures, drought, and wildfire.


When it is not possible to decommission roads, and/or remove road crossing structures, the following solutions should be considered, in the order listed.



Full-Span Bridge
Span the historically active floodplain or channel migration zone to minimize interference between infrastructure and channel processes, and optimize both terrestrial and aquatic species passage and full ecosystem function.

Stream Simulation Design (SSD)
Construct the following using approved SSD methodologies (in order of preference):

a. Bridge – Clear span
b. Bridge – With piers
c. Culvert
     i. Bottomless arch
    ii. Round pipe
   iii. Squash pipe
d. Modified SSD – Requires approval and waiver.

As a Last Resort: Hydraulic Design
This method is approved only when decommissioning and/or removing the road crossing structure is not feasible, or alternatives 1 and 2 (above) are unattainable. Hydraulic design styles include backwatered, embedded, and non-embedded culverts and culverts designed with a fishway or baffles.

Note: This solution needs sufficient planning/funding for annual, long-term monitoring and maintenance, for the life of the project.




North Fork Ryan Creek Inlet2_PAD 707086.

Click on the document to learn more about the background of the Fort Goff Creek Bridge.

Modeling Software

Full Channel Width Fish Passage Solutions

Upstream and downstream of barrier outside of any resulting facility scour, take two measurements of established channel widths.

For example:

  • Upstream: 22 feet and 19 feet

  • Downstream: 24 feet and 23 feet

  • The average of the widths is 22 feet.

  • 1.5 (22) + 5 = 38 feet

  • A full-span solution would be 38 feet or greater. Anything less is a partial, hydraulic solution.

Example of creek profile alignment sediment accumulation upstream and depletion downstream from scour
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