San Francisco Bay

Flood Size Increases Nonlinearly Across the Western United States in Response to Lower Snow-Precipitation Ratios

American Geophysical Union (AGU) | December 20th, 2019

Many mountainous and high-latitude regions have experienced more precipitation as rain rather than snow due to warmer winter temperatures. Further decreases in the annual

Many mountainous and high-latitude regions have experienced more precipitation as rain rather than snow due to warmer winter temperatures. Further decreases in the annual snow fraction are projected under continued global warming, with potential impacts on flood risk. Here, we quantify the size of streamflow peaks in response to both seasonal and event-specific rain-fraction using stream gage observations from watersheds across the western United States. Across the study watersheds, the largest rainfall-driven streamflow peaks are >2.5 times the size of the largest snowmelt-driven peaks. Using a panel regression analysis of individual precipitation and snowmelt events, we show that the empirical streamflow response grows approximately exponentially as the liquid precipitation input increases, with rain-dominated runoff leading to proportionately larger streamflow increases than snowmelt or mixed rain-and-snow runoff. We find that the response to changes in rain percentage is largest in the wettest watersheds, where wet antecedent conditions are important for increasing runoff efficiency. Similarly, the effect of rain percentage is larger across watersheds in the Northwest and West regions compared to watersheds in the Northern Rockies and Southwest regions. Overall, as a higher percentage of precipitation falls as rain, increases in the size of rainfall-driven and “rain-on-snow”-driven floods have the potential to more than offset decreases in the size of snowmelt-driven floods.

Flooded Wetland Availability for Breeding Waterfowl in a Mediterranean Climate: Mapping 38 Years of Historical Data in Suisun Marsh, California

University of California, Davis (UC Davis) | December 19th, 2025

Most managed wetlands in California are ephemeral and are purposefully flooded during the fall and winter for over-wintering waterfowl and are dry during the spring and s

Most managed wetlands in California are ephemeral and are purposefully flooded during the fall and winter for over-wintering waterfowl and are dry during the spring and summer waterfowl breeding season. Only semi-permanent and permanent wetlands remain flooded through the critical summer brood-rearing period for ducklings. We examined flooded wetland availability for breeding waterfowl in the brackish Suisun Marsh (California, USA) annually during the spring (April 27–May 17, during peak nesting) and summer (June 17–July 7, during peak duckling brood rearing), for a 38-year period using Landsat satellite imagery and spectral mixture analysis. Flooded wetland area increased 43% in spring and 48% in summer from 1984 to 2021 but varied among years (spring: 37.6–88.6 km2; summer: 17.7–57.5 km2). This increase in flooded wetland area over the past four decades was due to just a few sites, with only 24% (spring) and 15% (summer) of the 198 land-owner parcels increasing in flooded area. Flooded wetland area in the spring was unrelated to annual precipitation between October and April (range: 25–104 cm) or spring precipitation between January and April (range: 8-65 cm), whereas flooded wetland area in the summer was weakly correlated to both annual and spring precipitation. Flooded wetland area in spring and summer was also weakly correlated with the median daily outflow from the Sacramento-San Joaquin River Delta between March 15 and June 15, which corresponds to a critical period of wetland water management for breeding waterfowl. Our results indicate that spring and summer flooded wetland habitat for breeding waterfowl has slightly increased over the past four decades, varies annually, and is mostly dependent on local wetland management practices rather than on precipitation or Delta outflows. Managing habitats as semi-permanent wetlands would increase flooded wetland habitat in the spring and summer and provide habitat for nesting hens and ducklings.

Floods in California

Public Policy Institute of California (PPIC) | February 29th, 2024

Flow Augmentations Modify an Estuarine Prey Field

University of California, Davis (UC Davis) | June 15th, 2023

Flows and Fishes in the Sacramento-San Joaquin Delta

Delta Stewardship Council (Delta Council) | August 1st, 2015

Record-low counts of Delta smelt at a time of persistent drought underscore the importance and challenges of managing freshwater flows for the benefit of fishes in the Sa

Record-low counts of Delta smelt at a time of persistent drought underscore the importance and challenges of managing freshwater flows for the benefit of fishes in the Sacramento-San Joaquin Delta while also meeting human demands for water. Understanding the effects of water flows on fishes is central to understanding how the Delta ecosystem functions and is key to achieving the state’s coequal goals of “providing a more reliable water supply for California and protecting, restoring and enhancing the Delta ecosystem … in a manner that protects and enhances … the values of Delta as an evolving place”. The economic, ecological, and social costs of scientific uncertainty in water management controversies are significant - and to some degree unavoidable. Scientific findings that relate fishes and flows increasingly guide decisions on how to manage flows for the well-being of threatened or endangered species in the Delta. Many studies – and management decisions – rely on correlations between water flows and fish populations. But the decisions warrant fuller understanding of precisely how the flows affect the fishes. Knowledge of these underlying mechanisms is likely to facilitate adaptive management by clarifying uncertainty and risk, by creating specific expectations for outcomes and by strengthening testable hypotheses. This report therefore recommends, first and foremost (there are other recommendations as well), redoubling effects to identify causes and effects concerning fishes and flows in the Delta.

Flows and Fishes in the Sacramento-San Joaquin Delta Research Needs in Support of Adaptive Management

Delta Stewardship Council (Delta Council) | August 15th, 2015

Food Web Fuel Differs Across Habitats and Seasons of a Tidal Freshwater Estuary

Estuaries and Coasts (Springer) | July 11th, 2020

Estuarine food webs are fueled by multiple different primary producers. However, identifying the relative importance of each producer to consumers is difficult, particula

Estuarine food webs are fueled by multiple different primary producers. However, identifying the relative importance of each producer to consumers is difficult, particularly for fishes that utilize multiple food sources due to both their mobility and their generally high trophic levels. Previous studies have documented broad spatial differences in the importance of primary producers to fishes within the Upper San Francisco Estuary, California, including separation between pelagic and littoral food webs. In this study, we evaluated the importance of primary producers to adult fishes in three closely spaced subregions that represented disparate habitat types (a tidal wetland channel, a turbid backwater channel, and a deep open-water channel), each a potential outcome of local restoration projects. Using stable isotope analysis coupled with a Bayesian mixing model, we identified significant differences in primary-producer contribution to fishes and invertebrates across habitats and seasons, especially in the relative contribution of submersed aquatic vegetation and phytoplankton. Most fishes utilized multiple primary producers and showed little segregation between pelagic and littoral food webs among habitats. Availability of primary producers differs seasonally and across multiple spatial scales, helping to buffer environmental variability and thus enhancing food web resilience. Ecosystem restoration may improve with emphasis on restoring a wide variety of primary producers to support consumers.

Food Webs of the Delta, Suisun Bay, and Suisun Marsh: An Update on Current Understanding and Possibilities for Management

University of California, Davis (UC Davis) | October 1st, 2016

This paper reviews recent research findings on food web processes since 2008. It also highlights the idea of a changing baseline with respect to food web function. New re

This paper reviews recent research findings on food web processes since 2008. It also highlights the idea of a changing baseline with respect to food web function. New research indicates that interactions between habitat-specific food webs vary across the current landscape. For example, based on early work in the south Delta, the food web associated with underwater vegetation was thought to provide little support to species of concern; however, data from other regions of the estuary suggest that this conceptual model may not apply across the entire region. Habitat restoration has been proposed as a method of re-establishing historic food web processes to support species of concern. Benefits are likely for species that directly access such restored habitats, but are less clear for pelagic species. Several issues require attention to further improve the knowledge of food webs needed to support effective management, including: (1) synthesis of factors responsible for low pelagic biomass; (2) monitoring and research on effects of harmful algal blooms; (3) broadening the scope of long-term monitoring; (4) determining benefits of tidal wetland restoration to species of concern, including evaluations of interactions of habitat-specific food webs; and (5) interdisciplinary analysis and synthesis. The only certainty is that food webs will continue to change in response to the changes in the physical environment and new species invasions.

Fortresses of mud: how to protect the San Francisco Bay Area from rising seas

Nature Portfolio (Springer Nature) | October 9th, 2018

There’s something apocalyptic about this pond on the east side of San Francisco Bay, California. The legacy of a salt industry that has moved elsewhere, it has subsided

There’s something apocalyptic about this pond on the east side of San Francisco Bay, California. The legacy of a salt industry that has moved elsewhere, it has subsided a couple of metres below the level of neighbouring marshland. Algae paints red swirls in the brown water, and the pond’s edge is crusted hard with sparkling salt. As a breeze eases off the bay, a squadron of pelicans sails by, en route to more-appetizing hunting grounds. But there is a better future ahead for landscapes like this one in the Eden Landing Ecological Reserve and elsewhere around the bay. Over the next decade, government officials plan to fill many such depressions with sediment and then open them up to the tides. Eventually, cordgrass, pickleweed and other marsh vegetation will take root, restoring this crucial marsh ecosystem. The goal is to try to create a natural buffer to protect the heavily populated waterfront, by sapping energy from storm surges and blocking the highest tides.

Fourth National Climate Assessment Chapter 25: Southwest

U.S. Global Change Research Program | November 23rd, 2018

The Southwest region encompasses diverse ecosystems, cultures, and economies, reflecting a broad range of climateconditions, including the hottest and driest climate in t

The Southwest region encompasses diverse ecosystems, cultures, and economies, reflecting a broad range of climateconditions, including the hottest and driest climate in the United States. Arizona, California, Colorado, New Mexico, Nevada, and Utah occupy one-fifth of U.S. land area, extending across globally unique ecosystems from the Sonoran Desert to the Sierra Nevada to the Pacific Coast. The region is home to 60 million people, with 9 out of 10 living in urban areas and the total population growing 30% faster than the national average. The Nation depends on the region for more than half of its specialty crops such as fruits, nuts, and vegetables. The Southwest also drives the U.S. technology sector, with more than 80% of the country’s technology capitalization located in California. Click here for all chapters of the Fourth National Climate Assessment.