Nacimiento Dam Plunge Pool Protection Reaches 30% Design; County Targets Final Plans in November 2027

NACIMIENTO / LAKE NACIMIENTO AREA — October 30, 2026. Monterey County’s plan to protect vulnerable ground beneath Nacimiento Dam’s spillway has reached a measurable design milestone. In its September 17, 2026 briefing, the Monterey County Water Resources Agency reported that engineers had completed the 30% design package on August 28 and submitted it to the California Division of Safety of Dams for review on September 14. The agency plans to move into 60% design in November 2026, toward final design in November 2027, with a potential construction notice to proceed in June 2028 if permits, funding and contracts are secured.

These milestones describe engineering and planning, not a completed protective structure or a declaration that the dam is currently unsafe. A conceptual construction timetable is not the same as a funded, awarded building contract.

What is the plunge pool?

When excess water passes over a dam’s spillway, it enters the area below at considerable velocity. The plunge pool and adjoining banks must withstand those high-energy flows. The agency’s September 17 report, JBL 26-020, says past spill events eroded both the left and right banks, moved protective riprap and exposed weaker underlying rock. The proposed erosion-protection project is intended to reduce damage during severe discharge events, including design evaluations for extremely large floods.

Engineering consultants evaluated hydraulic alternatives and selected a preferred approach, Alternative 1, on February 19, 2026. It calls for a roller-compacted concrete erosion-protection system in the vicinity of the spillway plunge pool. Final dimensions and detailed plans remain subjects for design and regulatory review.

Completed work and remaining stages

In the county’s detailed chronology, topographic and bathymetric surveying took place in January 2024; geotechnical investigations ran from November 2024 into July 2025; alternatives modeling continued into February 2026; and the 30% design report was completed August 28, 2026. The agency said it had submitted that early design to state dam-safety regulators on September 14.

The same report lays out subsequent milestones as targets, not accomplished work: 60% design completion in January 2027, regulatory review in March–May 2027, 90% design by July 2027, final design in November 2027, construction bid advertisement in March 2028, and a possible notice to proceed in June 2028. Delays or changes to permitting, engineering and public funds may alter the sequence.

Environmental work is a separate step

The county says it engaged GEI Consultants in September 2026 for environmental review and permitting coordination, while AECOM Technical Services worked on hydraulic and design studies. The August 17 agency proposal described environmental services for both the Nacimiento plunge-pool project and a separate San Antonio Dam spillway project. These are two different dams and two different construction efforts, even though they were included in the same consultant-procurement process.

Review could involve the California Division of Safety of Dams, Federal Energy Regulatory Commission, regional water-quality regulators, California Department of Fish and Wildlife and the U.S. Army Corps of Engineers. The project’s 30% design and preliminary environmental work do not constitute final authorization to begin construction.

What this means for local residents

Nacimiento Reservoir supports regional water operations, recreation and downstream water management. Protecting dam infrastructure matters to residents across those service areas, but the plunge-pool project is not a public boat-ramp project and does not itself change the invasive-mussel inspection rules or floating-dock licensing already covered in Nacimiento News. Nor has the county’s September report issued a new evacuation order or established an imminent failure. For real-time conditions, consult the agency’s reservoir readings and the relevant county public-safety notices.

Questions for subsequent reporting include the final engineering estimate, which permits and mitigations will apply, whether an actual construction contract is awarded and how work staging might affect dam service access. None can be answered from a 30% design alone.

Official sources

Monterey County, JBL 26-020, September 17, 2026 design and permitting milestones; WRAG 26-252, August 17 environmental-services proposal. Published October 30, 2026. Projected dates are targets, subject to engineering, agency permits, funding and procurement.

Send verified updates to California58 Newsroom.

Featured photo: illustrative flowing water by Zack Dutra / Unsplash. Not the Nacimiento Dam spillway, the 30% design or a completed erosion-protection structure.

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