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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.

Winter Weather: Atmospheric Rivers

In California, water year 2017 has been an active year for atmospheric river (AR) storms. These storms are generally long and narrow, and carry tremendous amounts of water vapor, bringing large amounts of relatively warm, moist air. The precipitation that results when these storms make landfall can result in heavy rains and substantial snowfalls. The intensity of a single, very strong storm can trigger floods, landslides, avalanches, coastal erosion, and debris flow that endanger lives, property, and fragile ecosystems. Typically, ARs are strongest in the winter and early spring. Winter is normally the rainy season in California, and in early 2017 a sequence of AR storms arrived from over the Pacific Ocean. California felt the full force these storms, which brought above-average precipitation. This has contributed to above-average streamflow and snowpack, and encouraged flooding events throughout the state. Streamflow data collected by the California Streamgage Network provides real-time data on rivers and streams that is crucial to mitigating hazards associated with floods, flash floods, and debris flow.


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 400 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.