Mill Creek Dam Fish Passage

Mill Creek is a major tributary to the Russian River and harbors some of the best habitat for coho salmon and steelhead trout in the watershed. Fish access to 11.2 miles of excellent spawning and rearing habitat was severely blocked for decades by a 5-ft tall concrete flashboard dam that the National Marine Fisheries Service identified as “the highest priority barrier within the Russian River (coho) population for remediation.” Located in the backyard of several residential properties, the project required years of planning and negotiation with neighboring landowners.

PCI worked closely with Trout Unlimited (TU) to plan the project and secure funding for all phases from the NOAA Restoration Center, Sonoma County Water Agency, and California Department of Fish and Wildlife. Navigating significant site constraints, neighbors’ needs, and adult and juvenile fish passage requirements, PCI designed and constructed the project with the following components:

  • Side channel excavated into the hillside with a 3% grade for 100 ft.
  • A 6-8% grade, roughened boulder channel fill in mainstem to dam crest for 100 ft.
  • Dam elevation lowered 6 in.
  • Side channel entrance set 6 in. lower than dam
  • Roughened boulder-cobble-gravel channel throughout (4 to 6-ton boulders forming bed framework and roughness elements)
  • Shotcrete used for bank scour protection and inlet weir control, and sculpted to mimic natural rock

PCI, along with NOAA and CDFW, has conducted post-construction monitoring to evaluate passage conditions at a range of flows. UC Cooperative Extension and California Sea Grant continue to monitor salmonid use, and documented coho salmon spawning above the former barrier two months after work was completed.

The American Society of Civil Engineers presented the project team with a special Honorable Mention for the Distinguished Project Award at the 2017 International Conference on Engineering and Ecohydrology for Fish Passage.

The site before construction.

The site after construction.