Summer Report
This report shows trends in water quality, plankton, and fish across multiple IEP surveys for June through August from 1966 to 2025.
Delta Outflow
- Freshwater flow influences water quality, plankton, and fish populations.
- Summer flow is driven primarily by upstream dam releases and exports.
- In 2025, net Delta outflow was lower than the long-term average.
Figure 45: mean is represented by a dotted red line
Figure 46: Graph of net Delta outflow from 1966 to 2025. Values range from 2.6 to 46.1.
Secchi Depth
Background
- Organisms in this ecosystem are adapted to high turbidity conditions, and reductions in turbidity can have many negative ecological effects.
- Higher values for Secchi depth indicate lower turbidity.
- Secchi depth is measured monthly by DWR’s Environmental Monitoring Program by dropping a black-and-white disk in the water until it disappears.
Figure 47: image of a secchi disk
Average Secchi Depth by Region
Figure 48: mean is represented by a dotted red line
Figure 49: missing data are represented by tan triangles
Figure 50: Graph of average summer in San Pablo from 1976 to 2025. Values range from 33.8 to 228.2.
Figure 51: Graph of average summer in Suisun from 1975 to 2025. Values range from 23.2 to 70.5.
Figure 52: Graph of average summer in Delta from 1975 to 2025. Values range from 32.7 to 157.7.
For more information see: Schoellhamer, D. H. 2011. Sudden clearing of estuarine waters upon crossing the threshold from transport to supply regulation of sediment transport as an erodible sediment pool is depleted: San Francisco Bay, 1999. Estuaries and Coasts 34(5):885-899. https://link.springer.com/article/10.1007/s12237-011-9382-x
Water Temperature
Background
- Water temperature is monitored monthly by DWR’s Environmental Monitoring Program.
- High temperature can increase productivity and may trigger harmful algal blooms.
- Increasing Summer temperatures may limit juvenile smelt survival.
- Summer temperatures are lower closer to the ocean and slightly higher in the Delta.
Figure 53: picture of a thermometer in water
Average Temperature by Region
Figure 54: mean is represented by a dotted red line
Figure 55: missing data are represented by tan triangles
Figure 56: Graph of average summer in San Pablo from 1976 to 2025. Values range from 18.2 to 20.8.
Figure 57: Graph of average summer in Suisun from 1975 to 2025. Values range from 19.7 to 22.2.
Figure 58: Graph of average summer in Delta from 1975 to 2025. Values range from 20.8 to 24.1.
For more information see: Bashevkin, S. M., B. Mahardja, and L. R. Brown. 2022. Warming in the upper San Francisco Estuary: Patterns of water temperature change from 5 decades of data. Limnology & Oceanography. doi: https://doi.org/10.1002/lno.12057
Chlorophyll
Background
- Chlorophyll is an indicator of phytoplankton production, which is low during the summer.
- Phytoplankton are the base of the pelagic food web. It is sampled monthly by DWR’s Environmental Monitoring Program.
- The invasion of the clam Potamocorbula amurensis caused a decline in phytoplankton and zooplankton after 1986 – especially in Suisun Bay.
Figure 59: picture of phytoplankton
Average Chlorophyll by Region
Figure 60: mean is represented by a dotted red line
Figure 61: missing data are represented by tan triangles
Figure 62: Graph of average summer in San Pablo from 1976 to 2025. Values range from 1.2 to 22.1.
Figure 63: Graph of average summer in Suisun from 1975 to 2025. Values range from 1.2 to 28.4.
Figure 64: Graph of average summer in Delta from 1975 to 2025. Values range from 2.0 to 32.0.
For more information see: Brown, T. 2021. 2019 Phytoplankton Annual Report. IEP Newsletter 40(2):47-52.
Zooplankton
Background
- Zooplankton is sampled monthly by the CDFW/DWR Environmental Monitoring Program but sampling in San Pablo Bay did not begin until 1998.
- Zooplankton are an important food source for pelagic fish.
- Calanoid copepods and mysids are particularly good fish food. Cyclopoid copepods are not as good for fish food.
- Biomass in summer tends to be the highest of the year, particularly for the calanoid copepod Pseudodiaptomous forbesi.
Figure 65: picture of a copepod
Figure 66: picture of a mysid
Average Zooplankton Biomass by Region
Figure 67: mean is represented by a dotted red line
Figure 68: missing data are represented by tan triangles
Figure 69: Graph of total summer zooplankton biomass in San Pablo from 1998 to 2024. Values range from 1.7 to 10.9.
Figure 70: Graph of total summer zooplankton biomass in Suisun from 1975 to 2024. Values range from 5.2 to 44.7.
Figure 71: Graph of total summer zooplankton biomass in the Delta from 1975 to 2024. Values range from 5.5 to 40.6.
For more information see: Barros, A. 2021. Zooplankton Trends in the upper SFE 1974-2018. IEP Newsletter 40(1):5-14.
Fish
Background
- Delta Smelt, listed as threatened by the Endangered Species Act, have been tracked by CDFW’s Townet Survey since 1959 in Suisun Bay, San Pablo Bay, and the Delta.
- Northern Anchovy are an important forage fish in the brackish-saline regions of the estuary. They are sampled best by CDFW’s San Francisco Bay Study.
- Sacramento Pikeminnow is a native cyprinid that is one of the few piscivorous native fish in the Delta. They are sampled by USFWS’s DJFMP beach seine surveys throughout the estuary.
Average Fish Catch Trends by Species
Figure 72: mean is represented by a dotted red line
Figure 73: missing data are represented by tan triangles
Figure 74: picture of a delta smelt
Figure 75: picture of a northern anchovy
Figure 76: picture of a sacramento pikeminnow
Figure 77: Graph of juvenile Delta Smelt index from 1969 to 2025. Values range from 0.0 to 62.5.
Figure 78: Graph of average northern anchovy CPUE from 1980 to 2025. Values range from 170.5 to 1535.7.
Figure 79: Graph of Pikeminnow index from 1995 to 2025. Values range from 0.0 to 13.0.
For more information see: Malinich, T. D., J. Burns, J. White, K. Hieb, R. McKenzie, S. B. Slater, and A. Chorazyczewski. 2021. 2020 Status and Trends Report for Pelagic Fishes in the San Francisco Estuary. IEP Newsletter 40(3):75-97.
McKenzie, R. 2021. 2019–2020 Delta Juvenile Fish Monitoring Program - Nearshore Fishes Annual Report. IEP Newsletter 40(2):13-24.
Recent Trends
Background
- Delta Smelt have been in severe decline over the past two decades, with a Summer Townet Survey index of zero in 2015, 2016, 2018, and 2019.
- Microcystis is a toxic cyanobacteria first found in the Delta in 1998. Microcystis presence has been documented by the Environmental Monitoring Program and the CDFW Summer Townet Survey during their water quality sampling.
- Aquatic vegetation in the Delta has increased significantly in recent years. This vegetation is composed mostly of non-native invasive plant species and is categorized as either floating or submerged types. Coverage is estimated by UC-Davis using remote sensing of the North and Central Delta.
Average Trends by Parameter
Figure 80: mean is represented by a dotted red line
Figure 81: missing data are represented by tan triangles
Figure 82: picture of delta smelt
Figure 83: water with flakes of microcystis floating in it
Figure 84: stem of the plant egeria densa
Figure 85: Graph of juvenile Delta Smelt index from 2004 to 2025. Values range from 0.0 to 2.9.
Figure 86: Graph of microcystis abundance from 2007 to 2025.
Figure 87: Graph of percentage of water area occupied by aquatic vegetation from 2004 to 2022. Values range from 7.6 to 32.0.
For more information see:
Rasmussen, N., J. L. Conrad, H. Green, S. Khanna, J. Caudill, P. Gilbert, P. Goertler, H. Wright, K. Hoffmann, S. Lesmeister, J. Jenkins, L. Takata, D. Boswort, T. Flynn, E. Hard, and T. Sommer. 2020. 2017-2018 Delta Smelt Resiliency Strategy Action for Enhanced Control of Aquatic Weeds and Understanding Effects of Herbicide Treatment on Habitat, Interagency Ecological Program, Sacramento, CA. 332 pg.
Lehman, P. W., T. Kurobe, K. Huynh, S. Lesmeister, and S. J. Teh. 2021. Covariance of Phytoplankton, Bacteria, and Zooplankton Communities Within Microcystis Blooms in San Francisco Estuary. Frontiers in Microbiology 12(1184). doi: https://doi.org/10.3389/fmicb.2021.632264