The 77-Year-Old Dam Above Temecula Was Built for a Calmer Pacific

IMPORTANT: I'm publishing this as a draft to invite feedback and thought. This work is not finished and will change as I learn more.

The dam above Temecula is 77 years old.

In 2012, California state engineers determined its spillway cannot pass the largest flood a modern storm could deliver. Fourteen years later, the replacement is finally moving from paper into construction. It will not be finished before this coming winter.

And the models are now forecasting what could be the strongest El Niño in 140 years.

Most people in southwest Riverside County haven't connected these facts. They should.

The forecast

Meteorologist Chris Gloninger laid out the case in a recent piece at Weathering Climate Change. The European Centre for Medium Range Weather Forecasts, the most accurate seasonal model in operational use, is currently projecting sea surface temperature anomalies of roughly 2.5°C in the central Pacific by this fall.

For context: 1.5°C qualifies as a strong El Niño. The 1997 and 2015 events, the two largest on modern record, peaked near 2.3°C.

University at Albany atmospheric scientist Paul Roundy has publicly noted "real potential for the strongest El Niño event in 140 years."

Seasonal forecasts can shift between April and October. This one might. But the signal has held across four model cycles, and the subsurface heat pool in the western Pacific, which fuels these events, is unusually large.

What makes this cycle different from 1997 or 2015 is not El Niño itself. It's the baseline. Research published earlier this year found the pace of global warming has nearly doubled since 2015. The 1997 event occurred at 0.6°C above preindustrial. The 2015 event at 1.0°C. This one, if it develops, will occur at roughly 1.4 to 1.5°C.

That matters locally because a warmer atmosphere holds about 7% more water vapor per degree of warming. The atmospheric rivers that hit California this winter will carry meaningfully more moisture than the ones that hit in 1997 or 2015. This is true whether or not this specific El Niño materializes. It's the new operating condition.

The dam

Vail Dam sits about 15 miles east of Temecula on Temecula Creek. It's a 152 foot concrete arch built in 1949, owned since 1978 by the Rancho California Water District.

The state classifies it as "high hazard potential." In regulatory language, that means a failure would probably cause loss of life downstream. Per RCWD's own filings and the EPA WIFIA factsheet, the dam provides significant flood attenuation for Temecula Creek, including downstream Temecula and Camp Pendleton, though it was originally built in 1949 for irrigation and water supply rather than as a designated flood control structure. The CEQA Findings of Facts describe this as "passive flood control benefit."

The scale of downstream exposure is documented. The Findings of Facts, in rejecting the "No Project" alternative, state that the existing dam leaves 22,645 people at risk from catastrophic failure.

In 2012, the California Division of Safety of Dams (DSOD) found Vail Dam deficient on both seismic and hydrologic grounds. The seismic issue is what it sounds like. The hydrologic issue matters more for winter risk. According to Rancho Water's filings and the US EPA, the dam's spillway cannot pass the Probable Maximum Flood (PMF) without the reservoir overtopping the structure. The Draft EIR Technical Appendices quantify the deficiency: the PMF would overtop the existing dam by approximately 4 feet.

In plain language: during the theoretical largest possible storm, the spillway can't release water fast enough, the lake rises, and water goes over the top of the dam.

Worth some historical context here. Water behind Vail Dam has never overtopped the dam itself. It has spilled over the spillway structures during major rain events in the early 1980s (RCWD cites 1981, a federal Bureau of Reclamation document cites 1980) and again in 1993. The dam has also come through the 1997–98 super El Niño, comparable in magnitude to what's now forecast. What the 2012 finding identified is that it cannot handle the largest theoretically possible flood, which is a higher bar than anything in its operational record.

Since that finding, DSOD has imposed an elevation-based storage restriction. The operational cap is 1,457.6 ft NAVD88, roughly 15 feet below the spillway crest, corresponding to approximately 30,000 acre feet of storage. The restriction is expressed in elevation because that's what operators actually monitor.

Design storage capacity figures vary in the public record: RCWD's primary CEQA filings list 42,680 acre feet, while other sources give 45,000 or 51,000. The current operational cap is a meaningful reduction regardless of which design figure you use.

As of the December 2025 RCWD board packet, the reservoir was holding 12,160 acre feet at an elevation of 1,421.71 ft AMSL. Well below the cap. Well below design.

Where the project actually stands

The 2021 FEMA announcement of Phase 1 grant funding projected construction complete by mid-2025. The current target is Fall 2028. The project has slipped about three years.

What that schedule looks like in practice:

Design, CEQA, and permitting are complete. GEI Consultants was awarded the construction management contract in September 2024. The main remediation bid (solicitation D1911) was posted August 13, 2025, with bids due September 25, 2025. The solicitation is closed. By the time you read this, the prime contractor is likely already selected or close to it.

Heavy construction has not yet begun. As of the February 5, 2026 RCWD Finance and Administration Committee packet, paid-to-date on project D1911 was $8.64 million against a $101.5 million budget — about 8.5%. Open contracts are all professional services: AECOM, GEI, LSA, VCS, Verdantas, Trihydro, Blais. There is no prime contractor drawdown visible in the public record yet.

The honest framing is this: construction is poised to mobilize during 2026, with exact timing uncertain from public filings. The operational concern isn't the construction itself during this coming winter. The concern is the multi-year transition window between the old dam's operation and the new dam's commissioning, during which release capacity and freeboard management will be more constrained than during steady-state operation.

Funding is roughly 50/50 federal: approximately $47.5 million from FEMA's Hazard Mitigation Grant Program and approximately $42 million from the EPA WIFIA program. That the federal government is funding a $100 million-plus dam replacement is itself a measure of how seriously the state and federal dam safety apparatus take the deficiencies here.

What could go wrong

One thing to be clear about before going further. I am not a dam safety engineer, a hydrologist, or any kind of professional in the fields this section touches. What follows are scenarios drawn from the public documents cited below, not engineering analysis. Anyone making decisions about property, preparedness, or planning based on dam-related risk should consult Rancho California Water District directly, along with the licensed professionals overseeing the project.

With that said, the scenarios below track the categories of risk that professional engineers are already actively studying.

The moderate scenario is the most likely. A series of atmospheric rivers drops heavy rain on the 318 square mile drainage basin feeding Vail Lake. The reservoir rises toward its 1,457.6 ft elevation cap. Rancho Water begins forced releases down Temecula Creek to stay within the legal storage limit. Those releases arrive in the city during or just after storms that are already saturating soils, running Murrieta Creek high, and overwhelming street drainage.

Old Town Temecula currently has only 10-year flood protection while the Murrieta Creek Flood Control Project remains unfinished. Under this scenario, we see the kind of flooding the city saw in 1993 and 1998.

The more severe scenario is when the cap itself becomes the pinch point. If inflow during a multi-week atmospheric river sequence exceeds what the dam's existing outlet works can release, the lake rises above its restricted limit. At that point, the hydrologic deficiency DSOD identified in 2012 starts mattering in operational terms. You are asking a dam to pass a flood it has been explicitly assessed as unable to safely pass. The Technical Appendices put the magnitude at roughly 4 feet of overtopping at full PMF.

The tail scenario, low probability but not zero, is a dam safety event itself. Overtopping, spillway stress, or abutment failure during an extreme flood on a structure already flagged as hydrologically deficient. This is the scenario the "high hazard potential" classification exists to take seriously. It is not a prediction. It is the reason DSOD restricted storage in the first place, and the reason the dam is being replaced rather than patched.

The honest counterweight is that the dam has been through both the 1997–98 super El Niño and major spill events in the early 1980s and 1993 without overtopping. That operational record is real, and it's the reason the tail scenario sits in the low-probability category rather than somewhere higher.

Here is what gives these scenarios weight beyond my reading of the documents. AECOM, which Engineering News-Record has ranked as the nation's top dams and reservoirs firm, is the firm Rancho Water contracted to perform the Vail Dam Seismic and Hydrologic Deficiency Evaluation. Per AECOM's own Statement of Qualifications, their $4.35 million scope includes a Dam Breach Analysis using HEC-HMS and FLO-2D Pro software, an updated Emergency Action Plan with a new inundation map, an Interim Operation Restriction Plan, and evaluation of multiple remediation options including dam buttressing, dam notching, spillway modifications, and a new labyrinth spillway.

In other words: the top dam engineering firm in the country has already modeled what happens if this dam fails, has mapped which areas downstream would flood, and has written the interim restrictions that currently govern reservoir operation.

The scenarios above are not speculation. They are the same categories of risk that professional engineering studies are actively working to quantify and mitigate.

What to watch

Nothing here is a prediction. Dam operators, state engineers, and the construction team have managed this system safely within its restrictions for more than a decade. The elevation cap exists precisely to keep the risk envelope small.

But the atmosphere responds to forcing on its own schedule. The dam responds to water on its own schedule. Neither knows or cares about our readiness.

Three things worth tracking between now and winter:

1. The Niño 3.4 sea surface temperature index from NOAA's Climate Prediction Center. If anomalies climb through summer toward 2°C, the models are verifying.

2. Vail Lake surface elevation. The California Data Exchange Center station ID is VAI and provides real-time display (this is the source RCWD cites in its board packets). USGS station 11042510 provides historical data. The regulatory trigger is 1,457.6 ft NAVD88 — anything approaching that elevation means the lake is at its restricted cap and releases are imminent.

3. The Rancho Water construction schedule at ranchowater.com/VDCproject. Prime contractor mobilization, old-dam demolition phasing, and new-dam commissioning milestones will shape how constrained the operational window is across the next two winters.

If you live downstream of Vail Dam, along the Temecula Creek corridor, or anywhere the creek joins Murrieta Creek in Old Town, the practical steps are familiar. Clear your storm drains. Check your roof. Confirm your FEMA flood zone status. Remember that flood insurance has a 30-day waiting period, so that's a summer decision rather than a November one.

The forecast may soften. The winter may be ordinary. But the dam situation is real regardless of which way the Pacific goes, and the next twelve to twenty-four months are the window where attention pays off.

Sources and further reading

For anyone who wants to verify or dig deeper:

Chris Gloninger, Something Is Brewing in the Pacific That Nobody in Washington Wants to Talk About, Weathering Climate Change (April 17, 2026): https://chrisgloninger.substack.com/p/something-is-brewing-in-the-pacific

Rancho California Water District, Vail Dam Rehabilitation Project (schedule, scope, contractor updates, links to full CEQA record): https://www.ranchowater.com/472/VDC-Vail-Dam-Project

Primary CEQA record (Draft EIR, Technical Appendices, Findings of Facts, and MMRP). These are the authoritative regulatory documents, posted via the RCWD project page linked above. The Draft EIR Technical Appendices contain the PMF overtopping analysis (~4 feet); the Findings of Facts contain the 22,645-person downstream exposure figure and the 1,457.6 ft NAVD88 elevation restriction.

California Water Commission, Vail Dam Hazard Mitigation briefings (2019 documents confirming DSOD findings and storage restriction): https://cwc.ca.gov/-/media/CWC-Website/Files/Documents/2019/05_May/May2019_Agenda_Item_11_Attach_2_VailWriteUp.pdf https://cwc.ca.gov/-/media/CWC-Website/Files/Documents/2019/06_June/June2019_Item_8_Attach_1_VailOnePager.pdf

US EPA, WIFIA program page on the Rancho Vail Dam Seismic and Hydrologic Remediation Project (directly states the spillway is insufficient to pass the PMF without overtopping): https://www.epa.gov/wifia/rancho-vail-dam-seismic-and-hydrologic-remediation-project

AECOM, California Dams and Reservoirs Practice Statement of Qualifications. The Vail Dam project profile on page 35 documents the $4.35 million scope including Dam Breach Analysis, Emergency Action Plan with inundation map, Interim Operation Restriction Plan, and remediation options evaluation: https://aecom.com/wp-content/uploads/documents/brochures/AECOM-DCSA_CA-DamsReservoirs-SOQ.pdf

California Data Exchange Center, station VAI (real-time Vail Lake elevation): https://cdec.water.ca.gov/dynamicapp/staMeta?station_id=VAI

USGS monitoring station 11042510, Vail Lake near Temecula (historical elevation data): https://waterdata.usgs.gov/monitoring-location/USGS-11042510/

GEI Consultants press release on Vail Dam construction management: https://www.geiconsultants.com/press-release/vail-dam/

RCWD PlanetBids solicitation D1911 (Vail Dam Seismic and Hydrologic Remediation, posted August 2025, bids closed September 2025).

RCWD Finance and Administration Committee packets (for budget drawdown tracking; February 2026 packet referenced for 8.5% paid-to-date figure).

NOAA Climate Prediction Center, ENSO diagnostic discussions and Niño 3.4 index: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/

City of Temecula, Murrieta Creek Flood Control Project (current 10-year protection level, phased upgrades to 50-year and 100-year): https://temeculaca.gov/285/Murrieta-Creek-Flood-Control-Project

Corrections welcome. This piece leans on public records and the reporting of others. Where primary engineering documents disagree with secondary sources, the conflict is flagged rather than resolved.