Gradual spring reservoir releases match natural flows that support ecosystem health.

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: spring gradual recession flows in the Sacramento River.

  • Expectations

    Gradual spring recession flows on the Sacramento River support important ecological processes and restore a healthier Delta. 

  • Performance Metrics

    • Rate of change in the hydrograph on the receding limb as measured from spring high flows to summer low flows, evaluated annually and on a five-year rolling basis, at Bend Bridge on the Sacramento River.

Daily Change in Recession Flows

This chart shows the number of days between March 15 to June 1 where the daily flow recession is greater than 3.5 percent of the previous day. Target: no days with a daily flow recession rate greater than 3.5 percent. 

California streams and rivers decrease their flows in the spring during the end of the wet season. Gradual spring recession flow has wide-ranging effects on ecosystem health in rivers and is an important component of functional flows. Gradually receding flows are important for the avoidance of stranding of native fish and amphibians, success of many invertebrates that are prey to native fish, and establishment of key riparian plant species.

Appropriate recession flows are critical to the establishment of riparian tree species such as Fremont cottonwood, Gooding’s black willow, and sandbar willow. These trees help stabilize stream banks, produce debris that provides habitat for fish, and provide erosion control and shade.

Recession flow serves as a proxy for drawdown of the water level such that riparian tree species can access water and become established. Modeling indicates that rapid drawdown of up to approximately a week may be sustainable if followed by stable flows. Specific recession flows do not necessarily need to occur every year for these important riparian tree species. The target of 3.5% daily recession rate was selected because this rate can better support establishment of Fremont Cottonwood along the upper Sacramento River (Alexander et al. 2014). 

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
Metric:
  • Rate of change in the hydrograph on the receding limb as measured from spring high flows to summer low flows, evaluated annually and on a five-year rolling basis, at Bend Bridge on the Sacramento River.
Baseline:
  • Long-term hydrograph data from US Geological Survey gage station at Bend Bridge.
Target:
  • By 2030, daily decrease in flow will be less than 3.5% per day, as calculated by a five-day rolling average during the period of spring flow recession, in at least 1 out of 5 years, at Bend Bridge on the Sacramento River.

Bend Bridge USGS daily gage data (USGS gage 11377100)

Data from March 15 to June 1 for water each year 

Recorded numbers of days with a five-day rolling average greater than 3.5% decrease compared to the rolling average of previous day

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