Report Shows Science-Informed Operations Increase NorCal Resilience to Floods

Image caption: A drone panoramic view of Oroville Dam and main spillway at Lake Oroville in Butte County, Calif. Photo taken March 11, 2025. On this date, the water storage was 2,881,527 acre-feet (AF), 84 percent of the total capacity.

A new report released today shows that changes to reservoir operations at Lake Oroville and New Bullards Bar Reservoir can further reduce flood risk for communities along the Yuba and Feather rivers during extreme atmospheric river storm events and potentially benefit water supply during drier periods. The approach, known as Forecast-Informed Reservoir Operations, or FIRO, uses improved monitoring, weather, and runoff projections to build more flexibility and efficiency into reservoir operations.

In the largest FIRO assessment to date, the Department of Water Resources (DWR) and Yuba Water Agency (Yuba Water) partnered with the Center for Western Weather and Water Extremes (CW3E) at UC San Diego’s Scripps Institution of Oceanography and the U.S. Army Corps of Engineers’ (USACE) Engineering Research and Development Center to evaluate if FIRO could be implemented at both reservoirs to reduce downstream flood risk without negatively impacting water supplies. Lake Oroville is managed by DWR and New Bullards Bar is managed by Yuba Water.

Using historical forecasts, reservoir storage and river flow data, scientists found that FIRO, combined with a planned second spillway at New Bullards Bar, could provide additional flood storage capacity in the Yuba-Feather system and reduce downstream peak flows during prolonged storms like the 1986 and 1997 floods that devastated Yuba County. 

“California’s reservoirs play a critical role in safeguarding our communities from floods and maintaining our state’s water supply, especially as we continue to see more extreme weather events in the form of larger, wetter, and more frequent storms and longer, more severe periods of drought,” said DWR’s State Climatologist Dr. Michael Anderson. “The Yuba-Feather FIRO Viability Assessment is an important step toward improving flood protection to communities downstream while managing California’s valuable water resources for decades to come.”

The Yuba and Feather rivers originate in the Sierra Nevada and join at Marysville and Yuba City before flowing into the Sacramento River. With significant watershed runoff making both rivers prone to flooding, Lake Oroville and New Bullards Bar play a critical role in managing river flows, especially as the state sees more intense atmospheric river storms.

“Through the partnerships between research, forecasting, and reservoir operations organizations, the Yuba-Feather FIRO Viability Assessment has helped advance the integration of atmospheric river forecasting that enables FIRO to be viable at Oroville and New Bullards Bar,” said F. Martin (Marty) Ralph, founding director of the Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography. “CW3E's state-of-the-art weather model is tailored to atmospheric river forecasting and aided by extra weather observations offshore using aircraft and ocean observations as part of Atmospheric River Reconnaissance.”

While reducing flood risk is the top priority, FIRO can also improve California’s ability to provide a reliable water supply for communities and agriculture.

“This is a critical step towards making more floodwaters available for groundwater recharge when we see extremely wet storms,” said DWR Director Karla Nemeth. “FIRO drives tighter coordination between reservoir operators and water districts downstream, creating opportunities to use reservoir releases to build up our groundwater reserves for dry years.” 

Realizing The Full Benefits of FIRO With a Second Spillway at New Bullards Bar Dam

To fully realize the benefits of FIRO, Yuba Water is planning operational changes, continued atmospheric river monitoring, and the construction of a new Atmospheric River Control (ARC) Spillway at New Bullards Bar. The second spillway will have gates 31.5 feet lower than the dam’s existing spillway gates, allowing the agency to release water before large, threatening storms hit, when there is enough downstream channel capacity to handle the flows.

“The ARC Spillway will help realize the full benefits of FIRO in our region by reducing peak flows downstream and decreasing stress on our levee system during large atmospheric river events,” explained Yuba Water’s Director of Resource Planning John James. "With both FIRO and the ARC Spillway, we’re essentially gaining the amount of flood storage that would historically only be created through building additional infrastructure. In this case, we’re enhancing existing infrastructure and using the latest in science and technology to modernize flood operations and improve public safety.”

The ARC Spillway project is currently at 100 percent design with Yuba Water actively pursuing state and federal grant funding. Yuba Water anticipates construction could begin as soon as 2027. The existing spillway capacity at Oroville Dam is already adequate to fully realize the benefits of FIRO.  

Next Step: Incorporating FIRO Into Water Control Manual Updates

In the wetter winter months, both DWR and Yuba Water are required to draw down their reservoirs to ensure there’s enough flood storage space, known as the flood pool, to handle possible storms. These requirements are outlined in each reservoir’s water control manual, which USACE oversees.

Since advanced weather forecasting tools give dam operators more notice of incoming storms, water releases to reduce flood risk may begin earlier using FIRO, providing additional storage capacity to capture increased runoff into reservoirs. As storm systems pass and dry conditions return, releases from reservoirs can be reduced, potentially benefitting water supply.

Both agencies are working with the USACE Sacramento District to update their water control manuals. While FIRO and the water control manual are separate processes, the final viability assessment can help inform the water control manual process. The updates will ensure both reservoirs continue to comply with federal flood control guidelines while potentially adding flexibility to make operational decisions based on improved forecasting. Public meetings for the water control manual updates are on track to begin this year.

“The Yuba-Feather FIRO Viability Assessment, which the ERDC supported, is an important step toward formalizing our district's use of improved weather forecasts for strategic water management decisions at New Bullards Bar and Oroville reservoirs,” said Jenny Fromm, the USACE Sacramento District’s chief of water management. “As we consider including aspects of the report in water control manual updates for these non-USACE-owned reservoirs, public participation in the process will be another essential part of our efforts to decrease flood risk and improve drought resilience.”

FIRO is being implemented successfully at Lake Mendocino in the Russian River watershed in Northern California and will soon be integrated into operations at Prado Dam in the Santa Ana River watershed in Southern California. As the largest and most complex FIRO assessment to date and the first with a primary goal of reducing flood risk, the Yuba-Feather FIRO Program continues establishing FIRO’s benefit in California as an important part of a comprehensive water management and resilience strategy.