Yolo Bypass and the Fremont Weir

Near-term opportunities to reduce the conflict between ecosystem needs and water supply in the Sacramento-San Joaquin Delta are critical and limited.  The continued survival of endangered populations depends upon a balance between the two.  Typically, benefits to the ecosystem imply a negative impact on water supply, and vice versa. Clear scientific evidence, however, shows that creating inundated floodplain habitat on the Yolo Bypass would provide immediate benefits to endangered species in the Delta without compromising water supply availability.

The Yolo Bypass is a designated floodway that encompasses 60,000 acres in eastern Yolo County between the cities of Davis and Sacramento. Consisting of both public and private lands, all properties within the bypass are subject to a flood easement that allows the state to flood the land for public safety and ecological benefit.

The Fremont weir, a rudimentary structure approximately seven feet high and a mile long, controls inundation of the Yolo Bypass. Although the Yolo bypass floods more than once every two years, the timing and duration of inundation does not regularly create the benefits needed for covered fish species. Moreover, the lack of a functional fish ladder at the weir increases mortality of adult sturgeon and salmon which are attracted into the bypass by periodic flooding.

American Rivers has proposed a pilot plan for the Fremont weir to expedite habitat restoration and native species revival on the bypass. For more information, check out the video below:

American Rivers proposes a controlled notch in the weir that would:

  • Improve upstream passage of Chinook salmon, steelhead, and sturgeon
  • Optimize the timing and duration of inundation for covered fish species
  • Provide rearing habitat for endangered juvenile salmon and other fish, as well as critical spawning habitat for the declining Sacramento splittail
  • Increase the primary productivity of the Delta, which may be an essential step for restoring the endangered Delta smelt and other pelagic fish

Notching the weir is an ideal solution because:

  • Notching the weir is low cost, low risk, and easily reversible
  • Inundating the bypass is a high benefit, high certainty proposition
  • It has already been extensively studied and reviewed by scientists and policy makers

Delta Vision Blue Ribbon Task Force first recommended modifying the Fremont weir in the 2008 version of the Delta Vision Strategic Plan. Modification of the weir was also included in the National Marine Fisheries Service (NMFS) biological opinion for the Long-term Operations of the Central Valley and State Water Project, the NMFS recovery plan for salmon, and in preliminary drafts of the Bay Delta Conservation Plan.