Irvine Lake Improvement Project

Increasing water resiliency, enhancing safety, and improving efficiency

• Originally constructed in 1933, Irvine Lake, which is formed by the Santiago Creek Dam, serves as a vital part of Orange County water infrastructure.

• While Irvine Lake provides recreational benefits to the county, the primary purpose of the lake is to store water for the benefit of surrounding communities in Central and South Orange County.

• Irvine Lake/Santiago Creek Dam is jointly owned by Irvine Ranch Water District (75%) and Serrano Water District (25%).

• Today, the lake remains an important source of water for IRWD’s Baker Water Treatment Plant and SWD’s Walter Howler Filtration Plan, which collectively provide water to 85,000 homes throughout Central and Southern Orange County.


Beginning in 2017 and completed at the end of 2019, Irvine Ranch Water District and Serrano Water District undertook a comprehensive condition assessment of the nearly 90-year-old Santiago Creek Dam, along with its spillway and outlet tower. That assessment showed that while the dam is in good shape, the outlet tower and spillway are at the end of their useful life and should be replaced and upgraded to today’s seismic and safety standards.


The project will replace the current outlet tower and spillway at Irvine Lake/Santiago Creek Dam and restore Irvine Lake back to its full storage capacity. The project will be designed using a Risk Informed Decision Making process, that uses a rigorous, systematic, and thorough approach to dam safety that identifies and reduces risks. Incorporating RIDM into the project design is above and beyond the current standards required by the state and will yield an overall safer facility that will provide increased reliability for years to come.

Interested in learning more about dam safety? Watch the IRWD video: English | 中文字幕  | 한국어 자막 


Replace the existing outlet tower, which controls flow into and out of the reservoir, improve the regulation of flow and bolster the seismic performance of the structure.

• The new outlet tower will be embedded in bedrock within the slope and include multiple intakes, creating increased efficiency and seismic stability.

Construct a new spillway structure with nearly three times the original capacity.

• By increasing the spillway crest to 796 feet, we will increase the reservoir storage by roughly 3,700-acre feet, providing the opportunity for more water storage in the future.


Preserving and restoring local water supplies – The project will result in a significant amount of local water as a result of increased storage capacity, which will allow for more native water capture as opposed to purchasing expensive untreated water.

Increased water resiliency – The improvements at Irvine Lake will result in the additional storage capacity of 3,700-acre feet of water and ongoing opportunity for increased storage capacity in the future. This will result in enhanced water resiliency, allowing agencies to be less dependent on imported water.

Increased safety and reliability – The dam safety improvementsts, beyond the guidelines required by DSOD, will result in increased reliability into the future.

Improved efficiency and seismic performance – The new outlet tower will better control flow out of the reservoir and be designed for optimum seismic performance.

Promotes the well-being and security for the communities we serve – More than 1.5 million residents live in areas impacted, directly and indirectly, by the Santiago Creek Dam, and will benefit from the enhanced safety and seismic measures being implemented in the project. The project’s modern safety measures will protect residences, businesses, essential services, and critical fire/life safety infrastructure.


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Scoping Meeting presentation and flier (May 16, 2023)

Notice of Preparation and Scoping Meeting (May 16, 2023)

Initial Study (May 2023)