Delta Ecosystem Report Card

Progress to date toward a healthy Delta ecosystem has not been at a scale sufficient to slow or reverse the decline of native species and functioning habitats. Rather, ecosystem conditions, the status of native species, and water quality continue to deteriorate – while invasive species, hazardous harmful algal blooms affecting public health, and more are on the rise. Despite a recent increase in habitat acreage and completion of numerous restoration and multi-benefit projects, restoring ecosystem function is a slow, gradual process, and it may take many years for the ecosystem to respond positively to management actions and more time still to achieve the desired ecosystem outcomes. Reversing these declining trends remains a high priority that requires focus, resources, and perseverance.

The following Delta Ecosystem report card was developed as part of the Delta Plan Review that used the established performance measures to rate progress in implementing the Delta Plan over the past ten year (2013-2023). Read the 2024 Delta Plan Review full report to learn about additional review findings and the recommended actions that will inform Delta management priorities in next five years.

The overall performance measure score is rated “FAIR” (25-50% of target met) as of 2023 based on two metrics:

  1. 14-day consecutive inundation at a frequency of two out of three years: The target is Fremont Weir flows of at least 6,000 cubic feet per second (cfs) to provide sufficient inundation of the Yolo Bypass between November 1 and March 15. In wet years 2017, 2019, and 2023, the Yolo Bypass was inundated for 34, 30, and 23 days, respectively. Between 2013 and 2023, the frequency of 14-day inundation was achieved in four out of eleven years (36% of the time).
  2. 21-day consecutive inundation at a frequency of one out of two years, between November 1 and March 15: The Yolo Bypass was inundated for 30, 34, and 23 days, respectively. Between 2013-2023, five out of eleven years achieved the target (45% of the time).

View more: Yolo Bypass Inundation | Delta Stewardship Council (ca.gov)

The overall performance measure score is rated “GOOD” (51-75% of target met) as of 2023 based on two metrics:

  1. Dry and critically dry year outflow to inflow ratio: Between 2013-2023, the Delta outflow to inflow ratio was greater than 0.5 in every dry and critically dry water year (2013, 2014, 2015, 2020, 2021, 2022) (100% of the time). This means that at least half of the freshwater flow entering the Delta passed through the Delta into the San Francisco Bay during these dry years.
  2. Change in average annual outflow to inflow ratio across all water year types: This change is calculated as a ten-year trend (slope). The target is for the ten-year trend to be positive, meaning that Delta outflows increase proportionally to inflows. The Delta outflow-to-inflow ratio increased from 2013 to 2017 but decreased slightly from 2020 to 2022. This means there was a positive slope in five (2013-2017) out of the 10 years (50% of the time). In wetter years, 2017 and 2019, more than half of the freshwater inflow left the Delta (O/I=0.68). The average rate of change in the O/I ratio from 2013-2022 was positive (very slightly above zero (slope = 0.0035, p-value <0.001). Between 2013-2017, eight out of 10 years had an O/I ratio that was the same or greater than the previous year.

View more: In-Delta Flow | Delta Stewardship Council (ca.gov)

The overall performance measure score is “POOR” (0-25% of target met) as of 2023 based on metric:

  1. Frequency of 2-year return interval peak flows between November 1 and April 30, evaluated annually on a five-year rolling basis, at Bend Bridge on the Sacramento River. Between 2013 and 2023, the target of achieving at least one peak flow greater than 75,000 cfs and lasting at least 48 hours on the Sacramento River (at the Bend Bridge location) was met only in 2017. Every other year, there were insufficient days with high enough flows to reach the target.

View more: Peak Flow | Delta Stewardship Council (ca.gov)

The overall performance measure score is “POOR” (0-25% of target met) as of 2023 based on metric:

  1. 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: Between 2013 and 2023, the target of a gradual daily flow decrease (less than 3.5% per day) during the spring flow recession period (March 15 – June 1) on the Sacramento River at the Bend Bridge location was met only once in 2015. In every other year during this period, gradual changes in daily flows exceeded the flow threshold.

View more: Recession Flow | Delta Stewardship Council (ca.gov)

The overall performance measure score is rated “FAIR” (26-50% of target met) measured as the net change in ecosystem acres between 2007 and 2016 and compared to the 2030 interim targets for three metrics:

  1. Seasonal/Non-tidal wetland: Baseline area of 5,100 acres in 2007 increased by 6,117 acres (which is 97% of the Delta Plan 2030 interim target net increase from the baseline) to a total area of 11,217 acres of seasonal/non-tidal habitat in the 2016 vegetation mapping.
  2. Tidal wetland: Baseline area of 19,900 acres in 2007 decreased by 5,318 acres to a total area of 14,582 acres in the 2016 vegetation mapping. The Delta Plan 2030 interim tidal wetland target net increase from the baseline is 10,900 acres.
  3. Riparian Habitat: Baseline area of 14,200 acres in 2007 increased by 1,019 acres (which is 19% of the Delta Plan 2030 interim target net increase from the baseline) to a total area of 15,219 acres of riparian habitat in the 2016 vegetation mapping.

View more:  Acres of Natural Communities Restored| Delta Stewardship Council (ca.gov)

The overall performance measure score is rated “FAIR” (26-50% of target met) based on restoration projects completed between 2016 and 2022, and compared to the 2030 interim targets for the three major habitat types:

  1. Seasonal/Non-tidal wetland: Restoration projects during 2016-2022 completed 2,358 acres of seasonal/non-tidal wetlands (which is 37% of the Delta Plan 2030 interim target net increase from the baseline of 6,300 acres).
  2. Tidal wetland: Restoration projects during 2016-2022 completed 5,353 acres of tidal wetlands (which is 49% of the Delta Plan 2030 interim target net increase from the baseline of 10,900 acres).
  3. Riparian Habitat: Restoration projects during 2016-2022 completed 768 acres of riparian habitat (which is 14% of the Delta Plan 2030 interim target net increase from the baseline of 5,400 acres).

View more: Acres of Natural Communities Restored| Delta Stewardship Council (ca.gov)

The overall performance measure score is rated “POOR” (0-25% of target met) as of 2022 based on metric:

  1. Sacramento River watershed: The salmon runs in the Sacramento River watershed were 12% of the doubling goal. The current natural production is below the baseline.  

  2. San Joaquin River watershed: The salmon runs in the San Joaquin River watershed were 11% of the doubling goal. The current natural production is below the baseline

View more: Salmon Doubling Goal | Delta Stewardship Council (ca.gov)

The overall performance measure score is rated “POOR” (0-25% of target met) as of 2023 based on three metrics:

  1. Reduced invasive aquatic vegetation (POOR): Peak coverages of both submersed aquatic vegetation (SAV) and floating aquatic vegetation (FAV) have increased since 2015. SAV increased from around 7,500 acres under a 2004-2016 baseline to 12,000 acres between 2019-2020 (a 64% increase), with minimal differences by water year type. Floating aquatic vegetation (FAV) peak coverage increased to ~2,200 acres (a 12% increase) for both 2019 and 2020 compared to a similar baseline (~2,000 acres).
  2. Native fish population (POOR): The performance metric is expressed as a percentage of native fish population to non-native fish population from the overall fish biomass and abundance. The target for native fish biomass relative to non-native fish biomass was met in only one year (2023) between 2013-2023. Likewise, the target for native fish abundance relative to non-native fish abundance was met in only one year (2023) between 2013-2023. 
  3. Prevent new invasive species (POOR): The performance metric is expressed as a number of key non-native invasive species of fish, plants, and invertebrates established in the Delta. Between 2018 and 2023, at least three non-native invasive species were introduced in the Delta. These species are well-documented and likely to degrade the Delta ecosystem, waterways, recreation, and/or agriculture (i.e., Alligatorweed (Alternanthera philoxeroides), Ribbon Weed (Vallisneria australis), Nutria (Myocastor coypus)). Other non-native species were prevented from establishing in the Delta due to rigorous surveillance, prevention, and education, including Quagga/Zebra Mussels (Dreissena rostriformis bugensis/ Dreissena polymorpha), Water snakes (Nerodia sipedon), and Mute swans (Cygnus olor).

View more: Terrestrial and Aquatic Invasive Species | Delta Stewardship Council (ca.gov)