Science Stories: Adventures in Bay-Delta Data

Change is Afoot in the Delta
  • March 1, 2022

We all know climate change is going to be rough. We expect increases in temperature, changes in rainfall (where, when, and how much), and local extinctions or migration of plants and wildlife as the climate shifts. Climate change can sound abstract and is often spoken of as a phenomenon of the future, despite the changes we are already seeing in our surroundings. These changes affect the San Francisco Estuary and will eventually make it necessary to adjust the way we manage our water in California if we want to lessen the impact on those ecosystems. To better understand the impacts of climate change and to better inform management strategies, a group of Interagency Ecological Program (IEP) scientists wanted to find out how much is known about climate change in the Sacramento-San Joaquin Delta, Suisun Bay and Suisun Marsh and how management actions can lessen these effects. To do this, they gathered scientists with broad expertise – from zooplankton to aquatic vegetation – and created the Climate Change Project Work Team.

The team decided to start by creating a conceptual model (similar to the Baylands Goals model created for the San Francisco Bay) and synthesize already published research in a technical report. A conceptual model is an organized way of thinking through a particular problem, system, or idea in a visual way to make it easier to see and understand connections. Conceptual models are especially helpful when working in groups as while it is developed everyone participates and has to think through the problem and understands why the model looks like it does when it’s done. The climate change conceptual model made by the group let them see how the Estuary responds to different environmental drivers and that in turn showed what subjects to read about to find the answers they were looking for. The Climate Change conceptual model (Figure 1) started with global-scale changes in the top box, which impact landscape-scale environmental conditions in the Estuary. Those landscape-scale conditions influence site-level environmental change. For example, increases in global air temperature cause increases in water temperature in the rivers and bays, which in turn impact the temperatures experienced by each critter in the rivers. These climate-change effects also interact with landscape management (such as levee construction or wetland restoration) to impact the aquatic environment at a site.

Landscape impacts from climate change (for example, sea level rise, temperatures, and salinity field) impact local scale factors within an ecosystem.

Figure 1. The Climate Change Project Work Team's conceptual model.

Putting together the conceptual model and writing a synthesis of what we know so far is useful in other ways as well. It allowed the team to find out where there are things we need to study more if we want to be able to give better answers about what will happen in our aquatic ecosystems. The model highlighted three aquatic ecosystems in the estuary where organisms will experience different effects from climate change. The largest ecosystem in the Estuary today is open water. Marshes and floodplains make up a much smaller proportion of the habitat, but are still highly important to native species. Three different teams of scientists went on to review literature on the different ecosystems, diving into the current status of fish, benthic invertebrates, plankton and aquatic vegetation, and trying to predict changes and risks.

So, what did the teams find?

Out of the three, the open water ecosystem will be most impacted by drought and warmer temperatures. The changes brought by this will make this ecosystem more suitable for many of the invasive fish, invertebrates and aquatic vegetation, though higher salinity conditions during droughts may also favor some native fishes and aquatic vegetation (Figure 2). Predictions of future Delta temperatures have found that Delta Smelt's spawning window may be greatly restricted, further stressing this endangered fish (Brown et al. 2016).

Diagram showing current status of open water ecosystems, including invasive fish, weeds, and clams.

Climate change effects on open water ecosystems includes increased temperatures, increased invasive fish, and increased harmful algal blooms.

Figure 2. Impacts of climate change in open water ecosystems include harmful algal blooms, increased invasive clams, increased aquatic weeds, and increased invasive fishes, such as largemouth bass and Mississippi silversides.

Floodplains will experience major changes in timing and magnitude of inundation. Precipitation will become more variable with more frequent extreme floods and droughts. The larger storms we have seen lately benefit floodplains and the native fish that use them to spawn and feed, but only if they occur at the right time. Floods will shift to earlier in the season as more precipitation falls as rain instead of snow, keeping migratory species from being able to use the floodplain when they need it. More frequent droughts will mean the floodplain may not be available at all for years at a time (Figure 3). Management actions that increase the frequency or duration of floodplain inundation, such as the Yolo Big Notch Project, may become more important if floodplains are to be sustainable in the future.

Diagram showing current status of floodplains in the Delta. Most floodplain habitat is restricted to the Yolo Bypass and Cosumnes, but is important spawning and rearing habitat.

Aquatic fish and other aquatic life will have reduced use of the floodplains due to reduced frequency of inundation from extended periods of drought.

Figure 3. Floodplains, which are important habitat for spawning Sacramento Splittail and juvenile Chinook Salmon will not be inundated as frequently as droughts become more frequent, and may experience earlier flooding as more precipitation falls as rain instead of snow.

Tidal marshes are relatively scarce, but very important habitats. They provide food and nursery habitat for many fish and waterbird species. Whether they will continue to exist where they are will depend on the amount of sediment that will deposit in the marshes to keep up with sea level rise. Some models show that the larger storms will bring more sediment to the Delta which will help the marshes remain, but other models show that much of our tidal marsh will drown, especially if they do not have gentle, sloping transitions to uplands. Restoration planners may need to prioritize areas with adequate transition zones if they want restoration sites to be sustainable in the long-term.

Diagram showing current status of tidal wetlands in the Delta. Wetlands are relatively rare, but provide important rearing habitat with high food availability.

Tidal wetland size and functionality will be reduced due to sea level rise, increased temperatures, and invasive species.

Figure 4. Tidal marshes may drown as sea levels rise unless they have gentle transitions to upland areas. They may also experience the same increases to invasive species and increased temperature as open water ecosystems.

Other members of the Climate Change PWT have been working on looking for temperature trends from our monitoring record. They have found evidence for increased temperatures over the past 50 years (Bashevkin et. al., 2021), lower temperatures during wetter years (Bashevkin and Mahardja, 2022), differences in temperatures at the top and bottom of the water (Mahardja et. al., 2022), and hotter temperatures in the South Delta (Pien et. al., draft manuscript).

For a young adult audience interested to learn more about the San Francisco Estuary, the Sacramento-San Joaquin Delta in general and how climate change will affect it and the species living there check out a collection called Where the river meets the ocean – Stories from San Francisco Estuary . Many of the scientists that are on the team who wrote the Climate Change Technical Report also wrote for this collection, published by Frontiers for Young Minds.

Further Reading:

Bashevkin, S. M., and B. Mahardja. in press. Seasonally variable relationships between surface water temperature and inflow in the upper San Francisco Estuary. Limnology and Oceanography

Bashevkin, S. M., B. Mahardja, and L. R. Brown. 2021. Warming in the upper San Francisco Estuary: Patterns of water temperature change from 5 decades of data.

Brown, L. R., L. M. Komoroske, R. W. Wagner, T. Morgan-King, J. T. May, R. E. Connon, and N. A. Fangue. 2016. Coupled downscaled climate models and ecophysiological metrics forecast habitat compression for an endangered estuarine fish. Plos ONE 11(1):e0146724. 

Colombano, D. D., S. Y. Litvin, S. L. Ziegler, S. B. Alford, R. Baker, M. A. Barbeau, J. Cebrián, R. M. Connolly, C. A. Currin, L. A. Deegan, J. S. Lesser, C. W. Martin, A. E. McDonald, C. McLuckie, B. H. Morrison, J. W. Pahl, L. M. Risse, J. A. M. Smith, L. W. Staver, R. E. Turner, and N. J. Waltham. 2021. Climate Change Implications for Tidal Marshes and Food Web Linkages to Estuarine and Coastal Nekton. Estuaries and Coasts.

Dettinger, M., J. Anderson, M. Anderson, L. Brown, D. Cayan, and E. Maurer. 2016. Climate change and the Delta. San Francisco Estuary and Watershed Science 14(3).

Knowles, N., C. Cronkite-Ratcliff, D. W. Pierce, and D. R. Cayan. 2018. Responses of Unimpaired Flows, Storage, and Managed Flows to Scenarios of Climate Change in the San Francisco Bay-Delta Watershed. Water Resources Research 54(10):7631-7650. 2

Mann, M. E., and P. H. Gleick. 2015. Climate change and California drought in the 21st century. Proceedings of the National Academy of Sciences 112(13):3858-3859.

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