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Fish Passage Improvement Program (FPIP)

Willow Slough

photo of willow slough weir


Willow Slough Weir is an earthen dam constructed between 1924 and 1925 to control water levels in the East Borrow Canal (EBC) of the Sutter Bypass, in Sutter County, for agricultural diversions. The original structure had two 60-inch diameter corrugated metal pipe culverts and one 60-inch diameter concrete culvert that conveyed flow, controlled by slide gates, from the lower end of the EBC into Willow Slough; and a Denil fish ladder that was constructed in the 1980s to provide anadromous fish passage. The original fish ladder, which had a capacity of 90 cfs, had a number of structural issues and failed to provide the appropriate attraction flow when compared to the water being discharged through the culverts during much of the migration season. In addition, the existing culverts were undersized with respect to the planned operational flows from the EBC into Willow Slough.

Willow Slough Weir Project

The Willow Slough Weir component of the Sutter Bypass East Borrow Canal Water Control Structures Project was completed in 2010. The project included the removal of the existing fish ladder and culverts and the replacement of the original culverts with new structures whose design and configuration provide better flow control from the EBC into Willow Slough, and provide effective fish passage between the two waterways. Four new corrugated metal pipe culverts were installed to provide more flow capacity, and a new pool and chute fish ladder, which operates in a greater range of flows (6 to 270 cfs), was constructed through the weir. The fish ladder design was the alternative chosen after an analysis of conditions and needs at the site by DWR, Department of Fish and Game, and NOAA Fisheries staff and after input from other agencies and stakeholders.

DWR staff completed a preliminary engineering technical report (7.5 MB), funded by the Anadromous Fish Restoration Program, in 2005. FPIP staff prepared a mitigated negative declaration and initial study for the project in 2008. FPIP obtained environmental permits and approvals for the project.

The Willow Slough Weir falls within the Sutter Bypass Ecological Management Unit (EMU). One of the visions for the Sutter Bypass EMU is to ensure passage of chinook salmon migrating through the Sutter Bypass. Providing fish passage improvements at Willow Slough Weir will assist Bay-Delta agencies in meeting Milestones 67 and 71.

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