San Francisco Estuary and Watershed Science

The State of Bay–Delta Science (SBDS) is intended to inform science and policy audiences about the “state of the science” for topics relevant to management of the San Francisco Bay and Sacramento–San Joaquin Delta (“Bay–Delta”) system. When referencing the Bay–Delta system, we include the atmosphere, watershed, politics, and governance at a broad scale. Each SBDS edition has communicated new insights on a range of high-priority issues by synthesizing the current science and discussing progress on key research questions, knowledge gaps, and proposed future research. Collectively, these editions provide valuable summaries of the physical, biological, and social dimensions of the Bay–Delta. The first edition in 2008 provided a system-wide baseline on history, geography, water quality, ecosystem restoration, levee integrity, water supply, and public policy issues in the Bay–Delta (Healey et al. 2008). Eight years later, the second edition featured research on a dozen priority topics identified by senior scientists and managers working in the Bay–Delta (Healey et al. 2016), ranging from landscape change to migratory fishes to contaminants. Most recently, the third edition addressed research priorities identified in the 2017–2021 Science Action Agenda (DSC 2017), with a focus on the ecosystem services of primary producers (e.g., plants, algae, and their associated carbon) in the Bay–Delta (Larsen et al. 2023). Now, this fourth edition of SBDS focuses on governance and extreme events affecting the Bay–Delta: droughts, heatwaves, wildfires, and atmospheric rivers. The edition explores physical and ecological processes within the Bay–Delta that are responding to changes in large-scale forcing phenomena, primarily those associated with climate change, building on the rich long-term time-series data collected by regional and statewide monitoring programs.

Climate change affects nearly every aspect of the interdependent biophysical and social systems in California’s Sacramento–San Joaquin Delta. Mitigating and adapting to these effects will require effective climate governance: referring to the actors, rules, and processes through which decisions are made to prevent and respond to climate change. How governance systems effectively achieve these goals has become an increasingly central question in climate social science and climate policy debates, both at global and local scales. This paper reviews the state of science on climate governance in the Delta and investigates the extent to which effective climate governance characteristics operate in this region. The literature on climate governance broadly distills two key dimensions that scholars suggest influence efficacy: the structure of a governance system (e.g., extent of centralization and decentralization and mechanisms for coordination) and the degree of reactivity or proactivity in its processes. We review the available literature on Delta-specific governance, tracing the historical evolution of environmental governance in the Delta, and highlighting current efforts that illustrate different structural and procedural governance elements. Our synthesis finds robust evidence that characterizes the Delta’s governance system as dominantly polycentric and multi-scaler, increasingly participatory, and with a high aptitude for learning and innovation. Nevertheless, the region also faces key challenges around fragmentation and institutional fit, legacy policies that hamper transformational or proactive climate actions, and long-standing conflict among resource users and governing agencies. We conclude that the combination of high polycentricity alongside high levels of conflict and power asymmetries among affected parties in the Delta contributes to what can feel like “governance gridlock” and an inability to change the status quo to navigate new climate regimes equitably and effectively. These findings have implications for identifying steps forward for governance research and practice, both regionally in the Delta and beyond.

Droughts are frequent events in the western United States, and can disrupt water supply and degrade water quality, challenging water management in the Sacramento–San Joaquin Delta (Delta). This chapter for the State of Bay–Delta Science report describes what drought means for the Delta, how drought is managed in the Delta, and how drought management has changed over time. Projections of future climate indicate the possibility of increased frequency and severity of droughts which would have increasing effects on California’s water system, society, and ecological functions within and beyond the Delta. California has experienced several major droughts in the 20th and 21st centuries, each of which has caused significant social and ecological impacts and motivated improvements in water management. Droughts decrease native fish populations, increase harmful algal blooms, and promote the spread of many invasive plant and animal species. For people living within the Delta and those that rely on Delta water exports, droughts increase drinking water costs and decrease agricultural production, negatively affecting agricultural economies and labor markets. Tools developed in response to droughts include actions that increase supply, such as building water infrastructure, actions to reduce demand, such as water conservation campaigns, and mitigation actions, such as monetary relief for drought-impacted communities. Improving drought resilience requires development of additional drought responses, increased forecasting accuracy, and increased awareness of impacts on vulnerable communities and ecosystems. Even with development of additional management actions, strategies, and regulations, there will likely be difficulties meeting the current levels of demand for water. Drought conditions already cause conflict between human and environmental uses, and with more extreme droughts possible in the future and projected increases in demand, it will be challenging to provide for all users’ needs even with major changes to water management in the Delta.

Rising temperature is one direct consequence of climate change, and temperature is a key controlling variable on biological processes from molecular to ecosystem scales. While rising average temperature is one of the most discussed aspects of climate change, extreme events such as heatwaves are also expected to increase in duration, intensity, and frequency. These changes will bring about effects that threaten the integrity of the upper San Francisco Estuary (estuary) ecosystem, the services they provide to humans, and the health of humans that reside in the region. In the estuary, warmer temperatures are expected to result in seasonal shifts to life-cycle timing, and to favor smaller-bodied individuals across most non-human taxa. Several native fish species will likely decline, while a considerable number of non-native and cosmopolitan species tolerant of high temperatures are predicted to be relatively unaffected by or even benefit from a warmer climate. For humans, high temperatures and heatwaves are associated with wide-ranging health effects, from direct effects such as dehydration and heat exhaustion, to indirect and adverse health outcomes such as lower birth weight, mental health problems, and violence. These health effects will be exacerbated by ecosystem changes, as a longer warm season will increase our exposure to vectors such as mosquitos, as well as to the toxins produced by harmful algal blooms. Climate change is a global issue that cannot be resolved effectively at a regional level; however, some actions can either be taken or further studied to potentially lessen the effects of rising temperatures for the estuary’s ecosystem and residents at a more localized level. Although decreasing global greenhouse gas emissions remains our best option to combat climate change and the resultant temperature increases, successful adaptation to warming and heatwaves will require actions at multiple scales.

Estuaries provide critical habitat for many economically and ecologically valuable species that are adapted to a wide range of conditions and environmental variability, but the often turbid water presents challenges to monitoring efforts. This study explored fish habitat use in Richardson Bay, California (a sub-estuary of San Francisco Bay) at two points in time: one following a dry winter (2016) and the other following a historically wet winter (2017). Dual-frequency Identification Sonar (DIDSON) was used to record finfish and ray (>10 cm) abundance (MaxN) and size distribution, putative ray foraging pit size and abundance (MaxN), and eelgrass (Zostera marina) presence. We measured temperature, salinity, and dissolved oxygen (DO) at each site, and water samples at a subset of sites for nutrient analysis (urea, ammonium, nitrate, silicate, phosphate). Relationships between these data were explored using an information-theoretic modeling approach. Finfish abundance was best predicted by nutrient concentration in 2016 (–) and eelgrass presence in 2017 (–), whereas fish length was best predicted by salinity in 2016 (–) and eelgrass presence in 2017 (+). Foraging-pit abundance was strongly related to nutrient concentrations (+) in both years. This work presents a first attempt to establish relationships between fish distributions and environmental variables in Richardson Bay, and highlights the value of imaging sonar for studying fish communities in turbid estuaries.