Skip to main content
USGS - science for a changing world

California Flood Science

Monitoring Potential Flood Hazards

To help emergency managers and others prepare for floods, the USGS delivers a continuous source of streamflow information that provides the scientific basis for decision-making related to protection of life and property from water hazards. The USGS California Water Science Center maintains a network of nearly 500 streamgages that monitor hydrologic conditions throughout the State. Core data collected at streamgages are surface water levels that are used to determine the amount of water flowing in a river or stream. The USGS uses streamgage data to provide flood-warning alerts when surface water levels change rapidly, or reach flood-stage levels, which can signify potential hazardous conditions for downstream locations.

Atmospheric Rivers (AR’s)

Atmospheric rivers are long, narrow corridors moving through the atmosphere where water vapor is transported at high rates. These AR’s move moisture from the tropics in the Pacific over to the western coast of the United States. AR’s are responsible for great quantities of rain and have been identified to have a significant role in generating flooding across the western United States. They can also fuel strong winter storms that contribute to beneficial increases in snowpack.

Fall and Winter Weather: El Niño

El Niño is a naturally occurring event characterized by unusually warm ocean temperatures in the Equatorial Pacific. El Niño has come to refer to a whole complex of Pacific Ocean sea-surface temperature changes and global weather events. If El Niño conditions are present during the winter, the jet stream pattern over the U.S. shifts and can result in a wetter-than-average winter across the southern tier of the U.S., including portions of California.

On August 9, 2018, the National Oceanic and Atmospheric Administration Climate Predication Center issued an El Niño watch for the Northern Hemisphere. There is a ~60% chance of El Niño in the Northern Hemisphere fall 2018 (September-November), increasing to ~70% during winter 2018-19. Typical El Niño patterns in California include increased rainfall with accompanying floods, landslides, and coastal erosion. The effects are variable across the state and are more predictable in Southern California. For fall and winter 2018, NOAA’s temperature outlook suggests warmer than normal climate patterns and normal to wetter than normal precipitation patterns in California.


Flood Runoff in Relation to Water Vapor Transport by Atmospheric Rivers Over the Western United States, 1949–2015

Winter Weather in California Keeps CAWSC Field Crews Busy

Stormy weather: How the USGS goes to work monitoring its effects

Overview of the ARkStorm scenario

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

Streamgage Information

The U.S. Geological Survey has been measuring streamflow in the U.S. for over 120 years. We operate more than 7,500 streamgages in the U.S. and almost 500 in California.

Flood Activity

Current California flood alerts, including event summaries and streamgaging activity.

Debris Flow

Post-fire debris flows are a destructive landslide hazard. The USGS has conducted hazard assessments for select California wildfire burn areas.


A collection of USGS flood related photos, videos, news and publications.