Sacramento River

Antecedent moisture enhances early warning of atmospheric river flood hazards

Publisher not available | February 12th, 2026

Atmospheric rivers are narrow bands of water vapor transport and serve as key drivers of water supplies and flood hazards in mid-latitude regions. The atmospheric river s

Atmospheric rivers are narrow bands of water vapor transport and serve as key drivers of water supplies and flood hazards in mid-latitude regions. The atmospheric river scale supports early-warning communication by ranking events from potentially beneficial to hazardous based on atmospheric forcing represented by water vapor transport magnitude and duration. However, the scale does not consider land-surface conditions that can influence how precipitation derived from water vapor translates into streamflow and flood hazards. Analyzing atmospheric river landfalls across catchments in California and central Chile, we show that divergences between atmospheric river rank and flood response are primarily explained by pre-existing soil moisture conditions. Based on this insight, we develop a simple modification to the atmospheric river scale that nearly doubles the scale’s correspondence with peak streamflow and increases the number of flood-generating atmospheric rivers classified as hazardous by more than 30%. These findings demonstrate that incorporating land-surface conditions can enhance early-warning hazard classification tools.

Anthropogenic warming drives earlier wildfire season onset in California

American Association for the Advancement of Science (AAAS) | August 8th, 2025

Annual wildfire area in California has rapidly grown in recent decades, with increasingly negative impacts on people. The fire season is also lengthening, with an earlier

Annual wildfire area in California has rapidly grown in recent decades, with increasingly negative impacts on people. The fire season is also lengthening, with an earlier onset. This trend has been hypothesized to be driven by anthropogenic warming, but it has yet to be quantitatively attributed to climate drivers. Using a comprehensive fire occurrencedataset, we analyze fire season onset and climate controls on its variability and change during 1992–2020 in 13 California ecoregions. Northern California ecoregions show stronger trends toward earlier onset compared to more arid southern California ecoregions. Onset has trended earlier for all but one ecoregion. Interannual variability of onset is dominated by climate variability and its influence on fuel moisture. Trend attribution inferred from onset-climate relationships suggests that anthropogenic warming advanced fire season onset by 6 to 46 days during 1992–2020 in 11of 13 ecoregions. Continued warming is expected to further promote earlier fire season onsets.

Anthropogenic warming impacts on California snowpack during drought

American Geophysical Union (AGU) | March 15th, 2017

Sierra Nevada climate and snowpack is simulated during the period of extreme drought from 2011 to 2015 and compared to an identical simulation except for the removal of t

Sierra Nevada climate and snowpack is simulated during the period of extreme drought from 2011 to 2015 and compared to an identical simulation except for the removal of the twentieth century anthropogenic warming. Anthropogenic warming reduced average snowpack levels by 25%, with middle‐to‐low elevations experiencing reductions between 26 and 43%. In terms of event frequency, return periods associated with anomalies in 4 year 1 April snow water equivalent are estimated to have doubled, and possibly quadrupled, due to past warming. We also estimate effects of future anthropogenic warmth on snowpack during a drought similar to that of 2011–2015. Further snowpack declines of 60–85% are expected, depending on emissions scenario. The return periods associated with future snowpack levels are estimated to range from millennia to much longer. Therefore, past human emissions of greenhouse gases are already negatively impacting statewide water resources during drought, and much more severe impacts are likely to be inevitable.

Anthropogenic Warming Impacts on Today's Sierra Nevada Snowpack and Flood Risk

American Geophysical Union (AGU) | June 15th, 2018

This study investigates temperature impacts to snowpack and runoff‐driven flood risk over the Sierra Nevada during the extremely wet year of 2016–2017, which followed

This study investigates temperature impacts to snowpack and runoff‐driven flood risk over the Sierra Nevada during the extremely wet year of 2016–2017, which followed the extraordinary California drought of 2011–2015. By perturbing near‐surface temperatures from a 9‐km dynamically downscaled simulation, a series of offline land surface model experiments explore how Sierra Nevada hydrology has already been impacted by historical anthropogenic warming and how these impacts evolve under future warming scenarios. Results show that historical warming reduced 2016–2017 Sierra Nevada snow water equivalent by 20% while increasing early‐season runoff by 30%. An additional one third to two thirds loss of snowpack is projected by the end of the century, depending on the emission scenario, with middle elevations experiencing the most significant declines. Notably, the number of days in the future with runoff exceeding 20 mm nearly doubles under a mitigation emission scenarios and triples under a business‐as‐usual scenario. A smaller snow‐to‐rain ratio, as opposed to increased snowmelt, is found to be the primary mechanism of temperature impacts to Sierra snowpack and runoff. These findings are consequential to the prevalence of early‐season floods in the Sierra Nevada. In the Feather River Watershed, historical warming increased runoff by over one third during the period of heaviest precipitation in February 2017. This suggests that historical anthropogenic warming may have exacerbated runoff conditions underlying the Oroville Dam spillway overflow that occurred in this month. As warming continues in the future, the potential for runoff‐based flood risk may rise even higher.

Anticipating Environmental and Environmental-Health Implications of Extreme Storms: ARkStorm Scenario

American Society of Civil Engineers (ASCE) | July 22nd, 2015

The ARkStorm Scenario predicts that a prolonged winter storm event across California would cause extreme precipitation, flooding, winds, physical damages, and e

The ARkStorm Scenario predicts that a prolonged winter storm event across California would cause extreme precipitation, flooding, winds, physical damages, and economic impacts. This study uses a literature review and geographic information system-based analysis of national and state databases to infer how and where ARkStorm could cause environmental damages, release contamination from diverse natural and anthropogenic sources, affect ecosystem and human health, and cause economic impacts from environmental-remediation, liability, and health care costs. Examples of plausible ARkStorm environmental and health concerns include complex mixtures of contaminants such as petroleum, mercury, asbestos, persistent organic pollutants, molds, and pathogens; adverse physical and contamination impacts on riverine and coastal marine ecosystems; and increased incidences of mold-related health concerns, some vector-borne diseases, and valley fever. Coastal cities, the San Francisco Bay area, the Sacramento-San Joaquin River Delta, parts of the Central Valley, and some mountainous areas would likely be most affected. This type of screening analysis, coupled with follow-up local assessments, can help stakeholders in California and disaster-prone areas elsewhere better plan for, mitigate, and respond to future environmental disasters.

Anticipating responses to climate change and planning for resilience in California’s freshwater ecosystems

National Academy of Sciences (NAS) | July 29th, 2024

Applied Aquatic Science: A Business Plan for EcoAtlas

San Francisco Estuary Institute (SFEI) | March 7th, 2017

The EcoAtlas suite of tools represents a significant investment of time, energy, thought, scientific testing, technical innovation, and capital from a number of state and

The EcoAtlas suite of tools represents a significant investment of time, energy, thought, scientific testing, technical innovation, and capital from a number of state and federal public agencies, grant programs, and NGOs over the course of its 17-year existence. The toolset embodies the scientific and programmatic investments of the California Wetland Monitoring Workgroup (CWMW), its many related state and federal agencies, non-governmental organizations (NGOs), and private consultants, as well as the goals of the growing set of stakeholders who have expanded the circle of interested parties over the years. It is known by many names -- “WRAMP” being most common -- but in its essence it comprises the following tools:  ● EcoAtlas map viewer ● Project Tracker ● The California Aquatic Resources Inventory (CARI) map and editor tool ● The California Rapid Assessment Method (CRAM) ● The Riparian Zone Estimator Tool (RipZET) Now highly capable, the toolset represents more than science and technology alone, but a distillation of both, customized to meet specific goals associated with the landscape-scale tracking and characterization of California’s aquatic resources. The US Environmental Protection Agency (US EPA), for instance, has supported the toolset through multiple development grants so that the collected tools might be complementary to a still-nascent, statewide wetland protection program. Meanwhile, the state’s Coastal and Delta Conservancies require the use of the tool, and regional water boards -- including SF Bay, Lahontan, and North Coast -- regularly employ the tool for mitigation and restoration project tracking. Essentially, EcoAtlas has proven critical to a variety of programs and represents a successful product of broad-based collaboration. However, it currently stands at a crossroads, and its stakeholders must direct the toolset’s future. The following plan is intended to ensure the continued vitality of the toolset. The plan’s success will depend upon the continued collaboration of the public agencies that have supported the toolset thus far, but it must also integrate principles of resilience as it accounts for the tensions that arise as organizations move in different strategic directions. 

Aquatic Life Study Final Report

San Joaquin Valley Drainage Authority | January 6th, 2016

Central Valley Salinity Alternatives for Long Term Sustainability (CV‐SALTS) is developing a comprehensive regulatory and programmatic approach to the management of sa

Central Valley Salinity Alternatives for Long Term Sustainability (CV‐SALTS) is developing a comprehensive regulatory and programmatic approach to the management of salt and nitrate in the Central Valley that is consistent with the State Recycled Water Policy (SRWP). This work is being carried out collaboratively with the Central Valley Regional Water Quality Control Board (Central Valley Water Board), the State Water Resources Control Board (State Water Board), the Central Valley Salinity Coalition and Stakeholders. As stated in the CV‐SALTS Strategy and Framework document, the strategy to fulfill the requirements of the SRWP is to adopt a Central Valley Salt and Nitrate Management Plan (SNMP) and revise the Basin Plans applicable to the Central Valley to facilitate implementation of the SNMP. This effort includes a technical review of the state of science with regards to the establishment of appropriate water quality objectives (WQO) to protect surface water beneficial uses. This Aquatic Life Study was undertaken to evaluate the technical basis for the establishment of salinity‐related WQOs to protect aquatic life in the Central Valley. The findings of this analysis will be considered by CV‐SALTS during development and adoption of the SNMP.

Arsenic in California Drinking Water

Environmental Integrity Project | September 15th, 2016

More than three years after the U.S. Environmental Protection Agency found California in noncompliance with the federal Safe Drinking Water Act, 95 community water system

More than three years after the U.S. Environmental Protection Agency found California in noncompliance with the federal Safe Drinking Water Act, 95 community water systems in the state, serving more than 55,000 people, are still providing water with illegal levels of arsenic, according to an examination of state data for the last two years. Arsenic occurs naturally in the soil and groundwater in parts of California and is a known carcinogen that may also damage the developing brains of children and cause other health problems. Many of the people drinking excessive levels of arsenic are poor and/or Latino or African- American, with a cluster in the San Joaquin Valley. Nearly all have been exposed to excessive arsenic levels for at least five years and probably longer.

Assembly Bill 1337

State of California Office of Legislative Counsel | February 16th, 2023