Fish Passage Case Studies

20 Mile Creek
Culvert Replacement and Channel Re-grade with Step-Pool Morphology

Case Study Contributor
Rob Sampson, State Conservation Engineer, USDA-NRCS

Kootenai River, Idaho, USA

Project Type

  • Pre-fabricated Steel Bridge
  • Boulder step-pool grade control

Pre-Project Conditions

  • 6 ft (1.8 m) diameter corrugated metal pipe (CMP) at 3% slope
  • Cascade over riprap below culvert outlet
  • Aggraded and poorly-defined channel upstream of culvert creating forested wetland

Pre-Project Barrier

  • Bull Trout and other aquatic species blocked by culvert
  • Cascade down riprap at culvert outlet
  • Insufficient water depth and excessive velocities within the culvert

Watershed Characteristics

  • Drainage Area: 10 mi2 (25.9 km2)
  • Downstream Channel Gradient: 4%
  • High Passage Flow: 70 cfs (1.9 cms)
  • Low Passage Flow: 2 cfs (0.06 cms)
  • 2 year Return Flow: 138 cfs (3.9 cms)
  • Peak Design Flow: 25-year peak
    400 cfs (11.3 cms)

Ecological Value
Provide access to 1.5 miles (2.4 km) of low gradient habitat for adult Bull Trout (Salvelinus confluentus)

Project Characteristics

  • 40 ft (12 m) span prefabricated steel bridge
  • Step-pools consisting of 13 boulder weirs with 1.0 ft (0.3 m) drops spaced over 180 ft (55 m) of channel
  • Weirs constructed of 2 to 3 ft (0.6 to 0.9 m) diameter rock
  • Step-pool reach average slope of 7%
  • Water velocities, depths, and drop heights over weirs designed for passage of adult Bull Trout


  • Changing crossing type from box culvert to bridge to meet fish passage criteria
  • Undisclosed location of waterline, revealed during construction
  • Field adjustment of weir spacing, elevation, and drop heights
  • Temporary crossing for high-use residential road

Project Contributors

  • Natural Resources Conservation Service (NRCS )
  • Bonner Soil and Water Conservation District
  • Idaho Association of Conservation Districts

Project Funding

  • Boundary County Road and Bridge
  • Idaho Department of Environmental Quality (DEQ)
  • EPA 319 (Non point pollution control program)

Completion Date
October 2006

Project Cost

Project Summary

The old crossing was a complete barrier to Bull Trout due to a large drop over riprap below the culvert outlet. The replacement crossing was originally designed using a concrete box culvert. A step-pool bed form was designed to fit inside a 12 to 15 ft (3.6 to 4.5 m) wide box culvert. However, fish passage constraints created by the box culvert lead to the use of a prefabricated bridge with a 40 ft (12 m) span. To control the stream grade, a step-pool channel reach containing 13 boulder weirs placed over a 180 ft (55 m) of channel was designed. The upstream end of the step-pool reach was placed immediately upstream of the bridge.

The undersized culvert had caused substantial deposition upstream of the site, resulting in an ill-defined channel and floodplain and creating a small forested wetland. Originally there were discussions of constructing a ‘desired’ stream channel upstream of the site. The final design maintained the upstream channel grade using the boulder weirs. Removing the undersized culvert is anticipated to mobilize the upstream stored sediment, allowing the channel to seek its own location.

Analytical techniques were used to calculate water depths, velocities and shear stress associated with the step-pools. Bedload transport continuity though the reach was evaluated using the Meyer-Peter Muller approach. Published studies of step-pool channel morphology were used to determine weir spacing and drop heights. Water depths and velocities of the weirs were evaluated to ensure they satisfied Federal requirements for passage of adult salmonids, such as Bull Trout.

The final design included 13 weirs, each having 0.75 ft (23 cm) drops and spaced 11 ft (3.4 m) apart. Fine material was specified to be pressure washed into the gravel and boulder matrix to ‘seal’ the weirs and maintain surface flow. The project was allowed to be constructed within the flowing stream. Although this increases downstream turbidity, it makes it easier to determine if the weirs were sealed. A similar step-pool design was utilized on the Hell Roaring Creek project in an adjacent watershed.

Construction Challenges

A significant constraint was discovered about one hour prior to the rock placement. A County Road and Bridge worker revealed that a concrete water line crossed through the stream close to where the upstream weir was to be placed. There were no as-built records to locate the line so the entire suite of rock structures was moved about 20 feet downstream. This created substantial problems with the designed grade and required shortening the spacing and lowering the elevations of the rock weirs. The drop over the constructed boulder weirs was increased to 1 ft (0.3 m) and the overall slope of the step-pool reach was increased to approximately 7%.

The 20 Mile Creek Road is the sole access for many homes, making traffic control challenging. A portable bridge was used for a temporary crossing downstream of the construction site.


The site experienced a flow between a 10 and 25 year recurrence interval about 3 weeks after completion. No damage was observed. Hydraulics and recirculation patterns appear to look good from a fish passage standpoint.

Published 03/24/08