The State of Bay–Delta Science: An Introduction to the 2025 Extreme Events Edition

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.