California Assembly Bill 30, signed into law on September 1, 2023, is the state’s atmospheric river law that pairs storm forecasting with how reservoirs hold and release water. It repeals the earlier program created by Senate Bill 758 in 2015 and replaces it with a broader one codified at Water Code Section 347: the Atmospheric Rivers Research and Forecast Improvement Program, known as AR/FIRO. The Department of Water Resources runs the program, and its central job is to use better storm predictions to capture more water supply without raising flood risk.1California Legislative Information. AB 30 – Atmospheric Rivers: Research: Reservoir Operations
What the Law Requires DWR to Do
Section 347 directs the Department of Water Resources to research, develop, and implement new observation methods, prediction models, and forecasting techniques. The forecasts have to cover atmospheric rivers themselves and their downstream effects: water supply, flooding, post-wildfire debris flows, and environmental conditions.1California Legislative Information. AB 30 – Atmospheric Rivers: Research: Reservoir Operations
The statute also authorizes DWR to use the program’s research to refine climate projections of extreme weather and changes in Sierra Nevada snowpack. That matters because atmospheric rivers tend to arrive warm. They often melt existing snow rather than build on it, which accelerates runoff and compounds flood risk in mountain watersheds.
DWR’s duties don’t stop inside the department. Section 347 requires that information produced by the program be made available to relevant federal, state, and local agencies, so federal dam operators, local flood control districts, and emergency managers all get access to the same forecasts and analysis.1California Legislative Information. AB 30 – Atmospheric Rivers: Research: Reservoir Operations
Much of the underlying science comes through DWR’s partnership with the Center for Western Weather and Water Extremes at UC San Diego. Recent CW3E work on Sierra Nevada temperature forecasting has reduced forecast error by up to 1°C at short lead times and extended useful predictive skill by as much as two weeks at high elevations.
How Forecast-Informed Reservoir Operations Change Dam Management
FIRO is the operational core of the law. Traditional reservoir rules are rigid: operators must keep a fixed amount of empty space in a dam for flood control during winter months, whether or not any storm is coming. That approach protects against floods but wastes supply during dry stretches when nothing is on the horizon.
FIRO replaces those fixed rules with a flexible approach tied to the forecast. When no significant atmospheric river is expected within the next several days, operators can store more water than traditional rules allow. When a major storm is forecast, they release water ahead of time to make room. The whole strategy depends on predictions accurate enough, and far enough out, for operators to act on them.2Center for Western Weather and Water Extremes. FIRO Projects
Section 347 lists the goals of building FIRO into DWR’s operations: more water supply, more hydropower availability, and better overall supply reliability. The law directs DWR to reoperate flood control and water storage facilities specifically to capture water produced by atmospheric rivers and other storms.1California Legislative Information. AB 30 – Atmospheric Rivers: Research: Reservoir Operations
Where FIRO Is Already Being Used
Several California reservoirs are testing the approach. Lake Mendocino and Lake Sonoma in the Russian River basin were among the first FIRO pilot sites. The program has since expanded to Prado Dam and Seven Oaks Dam in the Santa Ana River basin, New Bullards Bar Reservoir and Lake Oroville in the Yuba-Feather River basins, and Howard Hanson Dam on the Green River in Washington state.2Center for Western Weather and Water Extremes. FIRO Projects
Many California reservoirs involve shared authority between state and federal agencies. The U.S. Army Corps of Engineers controls flood operations at numerous dams, and the Bureau of Reclamation manages water supply storage at major federal projects such as Shasta and Folsom. AB 30’s data-sharing requirement creates a formal pathway for DWR’s forecasting improvements to reach those federal operators.
The Army Corps has started building FIRO into its own operations. At Coyote Valley Dam on the Russian River, the Corps opened a public comment period for a water control manual update that would formally adopt forecast-informed procedures. The revised manual would allow operators to store up to 11,650 acre-feet of additional water in the flood control space based on five-day streamflow forecasts from the National Weather Service. The Corps tested those procedures through approved deviations during water years 2019 through 2026 before proposing the permanent rule change.3U.S. Army Corps of Engineers. Public Comment Period Begins for Coyote Valley Dam Water Control Manual Update
Local water districts sit at the other end of the information chain. When DWR’s models show a moderate atmospheric river approaching, a district might open diversion channels to capture runoff. When a stronger event is forecast, the same district shifts to protective mode and pre-releases stored water to create flood capacity.
What Changed From SB 758
California’s legislative work on atmospheric rivers started in 2015. Senate Bill 758 created the original Atmospheric Rivers: Research, Mitigation, and Climate Forecasting Program inside DWR, focused on developing the science to support flood and water management planning, forecasts, and warnings.4Center for Western Weather and Water Extremes. California Atmospheric River Program Theme
AB 30 kept the research mission and added an operational mandate: use the improved forecasts to change how reservoirs actually hold and release water. The newer law also broadened the program’s scope to include post-wildfire debris flow predictions and Sierra snowpack projections, recognizing that atmospheric rivers interact with other climate-driven hazards in ways the original program didn’t fully address.
Connection to the California Water Plan
The California Water Plan is the state’s long-term strategy for managing water resources. Under Water Code Section 10004, DWR must update the plan on or before December 31, 2028, and every five years after that.5California Legislative Information. California Water Code 10004 The plan tracks the status and trends of water supplies, agricultural and urban demand, and environmental water needs across a range of future scenarios.
As DWR’s forecasting capabilities improve under AR/FIRO, data on precipitation patterns, flood risk, and water capture potential feeds into the assumptions underlying the plan’s projections. The most recent update, completed in 2023, addressed sustainable management and stewardship of California’s water resources, including climate adaptation strategies that depend on the kind of science AB 30 funds.6Department of Water Resources. California Water Plan Update 2023 Each five-year update can incorporate the latest atmospheric river data, refined climate projections, and lessons learned from the FIRO pilots.