Sufficient seasonal inundation of the Yolo Bypass supports native fish species.

The Measure

Restoring to a healthier estuary using more natural functional flows - including in-Delta flows and tributary input flows - to support ecological floodplain processes: Yolo Bypass floodplain inundation.

  • Expectations

    More natural functional flow patterns are restored in the Yolo Bypass floodplain to support native fish spawning, and rearing along with important ecosystem processes. 

  • Performance Metrics

    • Area and duration of inundation in the Yolo Bypass, evaluated annually on a five-year rolling basis.

Number of Consecutive Days with Fremont Weir Flows of at least 6,000 cfs, 1997-2019

Sacramento River flows into the Fremont Weir can overtop and inundate the Yolo Bypass during high flow events. The target is Fremont Weir flows of at least 6,000 cfs for sufficient inundation  between November 1 and March 15This target is used as a proxy for inundation area because current model estimates of Yolo Bypass inundation is only up to water year 2012. The target also requires sufficient inundation for 14 consecutive days every two out of three water years and 21 consecutive days every one out of two water years. 


Targets Met

Target 1: 6,000 cfs for 14 consecutive days in two out of three years. Target 2: 6,000 cfs for 21 consecutive days in one out of two years. * Indicates target was not met in 1997 due to previous year's flows not being considered in the target. ** Indicates target was met that year due to previous year's flows, even though flows of that year did not meet the target.

Yolo Bypass is a large floodplain habitat adjacent to the lower section of the Sacramento River that is frequently flooded and provides alternate routing of flows and young fish through the Delta. Floodplain inundation provides key ecological functions and restoring more natural functional flow patterns in the Yolo Bypass delivers important ecological benefits to fish in the lower Sacramento River.

The Yolo Bypass has been extensively studied for its potential to contribute to the growth of migratory fish species and contribution to the Delta food web. It has been consistently identified as a high opportunity area for providing floodplain and wetland functions. Native fish species such as Chinook salmon, steelhead, and Sacramento splittail accessing inundated habitat of the Yolo Bypass have higher growth rates and consequently increased survival rates. Juvenile fish migrating downstream through the Yolo Bypass also keep away from the southern Delta where mortality rates are higher. Food production (phytoplankton and zooplankton) and juvenile fish growth generally increase with increased floodplain inundation duration, although there is uncertainty as to the time period and frequency of inundation that provide maximum ecological benefits to the fisheries. Current collaborative and adaptive management efforts will provide additional information to guide the restoration of more natural flow patterns in the Yolo Bypass.

Yolo Bypass Salmonid Habitat Restoration and Fish Passage Project               

DWR and U.S. Bureau of Reclamation developed the Yolo Bypass Salmonid Habitat Restoration and Fish Passage Project to improve fish passage and increase floodplain fisheries rearing habitat in Yolo Bypass and the lower Sacramento River basin. This mainly consist of a new Fremont Weir headworks structure, a new outlet channel, and downstream channel improvements. Anticipated construction of this Fremont Weir notch is in 2020 or 2021. The project is in the alternatives development phase, which includes conceptual engineering, impact analysis, and avoidance and minimization measures. A public draft Environmental Impact Statement/Environmental Impact Report (EIS/EIR) was released in December 2017 and a Final EIS/EIR was released June 2019.

Fremont Weir Adult Fish Passage Video (DWR)

Map of Yolo Bypass. Fremont Weir on northern end of the floodplain.
Map of Yolo Bypass. Fremont Weir on the northern end of the floodplain.

Each chapter of the Delta plan includes strategies to achieve the goals of the plan. These strategies are general guidance on achieving the objective laid out in the plan and in the Delta Reform Act of 2009. Associated with these strategies are recommendations. The recommendations describe more specific and implementable actions to support the achievement of Delta Plan strategies. Strategies and recommendations may also have associated performance measures. Delta Plan performance measures track progress in achieving desired outcomes for the Delta Plan. Below are the strategies and recommendations associated with this performance measure. 

Delta Plan Strategy:
  • Create more natural functional flows

Delta Plan Recommendation:
  • Update Delta flow objectives

Area and duration of inundation in the Yolo Bypass, evaluated annually on a five-year rolling basis.


Modeling for the years 1997–2012 estimates that events with a 14-day duration inundated 45,100 acres in 33 percent of years, 19,700 acres in 50 percent of years, and 16,400 acres in 67 percent of years. Events with a duration of at least 21 days are estimated to have covered 36,300 acres in 33 percent of years, 15,800 acres in 50 percent of years, and 10,000 acres in 67 percent of years, between November 1 and May 30 (DWR 2015).


By 2030, allow for at least 17,000 acres of inundation for at least 14 days in two out of three years and at least 21 days in one out of two years, between November 1 and March 15

Flow data for at the Fremont Weir gage station (DWR) measuring daily flow into the Yolo Bypass

Recorded highest consecutive number of days with at least 6,000 cfs for each water year

Data from November 1 to March 15 of each water year were used

6,000 cfs was used as the target for Fremont Weir flows because it simulates inundation sufficient to support native wildlife species  

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