Freshwater outflow from the Delta into the San Francisco Bay supports native 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: in-Delta flow.

  • Expectations

    An increase in the ratio of Delta outflows to Delta inflows contributes to restoring more natural functional flow patterns in the Delta.

  • Performance Metrics

    • 10-year rolling average slope of the Delta outflow to inflow ratio in seasonal, annual, and 10-year periods.
    • Outflow-inflow ratio in dry and critically dry years, evaluated annually on a five-year rolling basis.

Slope of Outflow to Inflow Ratio

This graph shows the slope of 10-year rolling averages of the Delta outflow to inflow ratio by decade. Target is a slope greater than 0.

Delta Outflow to Inflow Ratio during Dry and Critically Dry Years Only

The graph shows the ratio of Delta outflow to inflow for dry and critically dry water years. The target for the annual average Delta outflow to inflow ratio is 0.5 for dry and critically dry water years. 

Flows through the interior of the Delta, represented by the ratio of outflow from the Delta to the inflow into the Delta, are an important component of functional flows and have wide-ranging effects on the health of the Delta and the lower San Francisco Bay Estuary. The ratio of Delta outflows to inflows encompasses a vast array of flow and ecological complexity, and is relevant to in-Delta flow patterns, including the overall quantity, timing, and variability of flow regimes. Variability of flows between years is very high in California, therefore a ratio is used to account for different water year types. A higher outflow to inflow ratio contributes to restoring more natural functional flow patterns in the Delta.

Over the past century, land use change, construction of large-scale water management infrastructure, export of water, in-Delta diversions, and consumptive water use have greatly changed flow dynamics in the Delta. Several rivers, creeks, and other water bodies contribute water to the Delta, the largest of which are the Sacramento and San Joaquin Rivers. Freshwater inflow into and through the Delta greatly influences the ecological health of the Delta, as well as water quality and species abundances. Studies continue to evaluate ecosystem responses to Delta flows and how flow, non-flow, and other factors interact to impact specific species. 

As of late 2018, the State Water Resources Control Board (SWRCB) is proposing an update to the Bay-Delta Water Control Plan to improve flows to support a connected and functioning ecosystem. This proposal will impact the watersheds of the Sacramento and San Joaquin Rivers. The update calls for linking flow measures such as inflow, cold water habitat, Delta outflow, and interior Delta with non-flow measures such as improving habitat restoration, fish passage, and predator control. In December 2018, SWRCB adopted a plan designed to increase water flows through the Lower San Joaquin River and its tributaries—the Stanislaus, Tuolumne and Merced Rivers. 

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

(1) 10-year rolling average slope of the Delta outflow-inflow ratio, disaggregated by seasonal, annual, and 10-year periods, (2) outflow-inflow ratio in dry and critically dry years, evaluated annually on a five-year rolling basis.


Long-term ratio of Delta outflow to Delta inflow. The period before construction of the Central Valley Project and State Water Project and select major dams (1931-1954) had a Delta outflow-inflow ratio of 0.88. Post-completion of most components of the State Water Project (1981-2015), the Delta outflow-inflow ratio was 0.75.


By 2030, (1) 10-year rolling average slope of Delta outflow-inflow ratio is greater than zero (i.e. positive), and (2) Annual average Delta outflow-inflow ratio in dry as well as in critically dry years is greater than 0.5.

  • Ratio of the daily data of Net Delta Outflow Index (NDOI) and Total Delta inflow from the Department of Water Resources (DWR)

  • Calculated 10-year rolling averages and annual averages of the outflow-inflow ratio

  • Slopes of each decade starting with 1940-1949 were calculated

  • Water year hydrologic classification indices are based on the DWR Sacramento Valley index

Have questions or comments about this measure? Contact Us