Stanislaus dam controversy has it all: seismic concern, condors, unusual rocks
Gift Article
A peaceful canyon in western Stanislaus County, where people hike and enjoy the scenery, also is a battleground over a proposed reservoir just outside Patterson.
The U.S. Bureau of Reclamation released an environmental impact statement this month that’s likely to spark new debate about seismic activity around Del Puerto Canyon, the safety of the proposed dam and wildlife. In a new wrinkle, some dam opponents have noted that legally protected California condors have been flying not far from the reservoir site.
A spokesman for the state Division of Safety of Dams said the regulatory agency expects to see a construction application in 2026 from Del Puerto Water District and its reservoir partner, the San Joaquin River Exchange Contractors Water Authority, an association of four water districts.
Depending on approvals and funding, the six-year dam construction project could begin in 2027. The proposed reservoir on the west side of Interstate 5 would store 82,000 acre-feet of water and reach 4.5 miles into Del Puerto Canyon.
“The project is currently at 35% percent design for all facilities,” said Anthea Hansen, general manager of the Del Puerto Water District, adding the districts are working with utilities on relocation of overhead power lines and with the county on a $113 million realignment of Del Puerto Canyon Road.
The $1.2 billion reservoir is intended to benefit farmers who have not-so-solid contracts for irrigation water from the Central Valley Project, which consists of 20 reservoirs and 500 miles of canals. Simply put, it would store water in wet years, when contracts are mostly honored, and deliver water to farms in dry years when allocations are cut.
Some Patterson residents who traditionally love the canyon suspect there’s more to the project than that and don’t see how another reservoir will fix California’s problem of overcommitted water resources.
The reservoir also faces opposition from former Bay Area residents who moved here for less costly housing and discovered the canyon, as well as groups like Save Diablo Mountain that celebrate the natural wonders of the Diablo Range.
Elias Funez, a volunteer for Save Del Puerto Canyon, said the city of Patterson had park amenities planned for the canyon, including hiking and bike trails, going back to 2012. The reservoir will inundate a natural gem and recreational asset that’s beneficial to the health of the city’s growing population, he said.
“We are not against farmers, irrigation or water districts, but none of the project makes any sense,” Funez said. “You see people walking around in the canyon, hiking, taking pictures. It’s a chance for them to breathe fresh air and enjoy activities that are healthy.”
The Del Puerto Water District was the lead agency for a state-required environmental impact report completed in 2019, which was challenged in a lawsuit by the Sierra Club, California Native Plant Society, Center for Biological Diversity and Friends of the River. A Stanislaus Superior Court Judge sided with the water districts but ordered more study of the Del Puerto Canyon Road realignment.
The plaintiffs and Exchange Contractors each filed separate appeals and the Fifth District Court of Appeal ruled in January the EIR had neglected to study terrestrial species along Del Puerto Creek downstream from the dam site. The appellate court reversed on the road issue. The water district released an EIR addendum this month that identified no impacts on terrestrial species.
Funez said the draft federal EIS released this month is a cut-and-paste of the water districts’ EIR. New concerns have arisen since the 2019 environmental study, he said, including a September 2023 swarm of earthquakes shaking the foothills in the area of Diablo Grande and Westley.
While the county is not considered prone to major earthquakes, both environmental studies refer to faults in the region, as the folded hills and canyon geology are a result of tectonic forces on California’s coast over millions of years. The San Joaquin fault is identified half a mile east of the main dam site.
According to studies, failure of the dam would cause water to overtop Interstate 5 and possibly the California Aqueduct and flood parts of Patterson. The water district’s final EIR said a seismic hazard analysis will estimate potential levels of ground shaking. The dam facilities will be designed to provide “acceptable performance under a maximum credible earthquake,” the study said.
Condors expanding their range
KQED reported that six California condors have been tracked in the sky in the Mountain Diablo area and at Mount Oso in Stanislaus County, which is north of Del Puerto Canyon. The news report cited a condor program manager who said the protected birds at Pinnacles National Park, 125 miles south of Mount Diablo, were expanding their range and exploring other mountains in the broader region.
Conservation programs to save California condors from extinction have made progress in the past 45 years. Condors are not mentioned in the federal EIS on the reservoir project. Official sightings of the large birds, one of the most famous endangered species, could create headaches for Del Puerto dam proponents, who promise mitigation measures for adverse impacts on eagles and other birds.
Hansen said Thursday the districts’ EIR found no record of California condor sightings near the reservoir site.
Crucial geotechnic investigations
Experts from around the world have studied the canyon’s unusual geologic formations. Professors from local colleges offer field trips through the canyon to teach about the geology — how the sedimentary layers of the Great Valley Sequence were uplifted by the Franciscan Complex through subduction and the Coast Range Ophiolite (ocean crust and rocks from Earth’s mantle) are exposed in the upper canyon.
The proposed main dam and two saddle dams would be constructed in Great Valley Sequence geology, which consists of alluvial plain and a “thick accumulation of marine and nonmarine clastic rocks,” the districts’ EIR said. Clastic refers to fragments of rocks and minerals cemented together.
Project engineers have done geotechnical testing and borings for more than three years in trying to evaluate the bedrock underground that would form the Del Puerto dam foundation, its ability to withstand seismic pressure and whether it’s substantial enough to prevent seepage. Seepage occurs with all dams, and an effective seepage control system is needed so the Del Puerto dam is not compromised.
“With regard to how solid and secure are the dam and abutment sides, that is clearly an issue if you are siting a dam in the Coast Range,” said Ron Stork, senior policy staff member with Friends of the River. “In California, we have had failures along abutments before.”
Andrew Dinsick, deputy project manager for the reservoir project, said when he first visited the site in 2021, it looked like the ideal spot for a storage reservoir, as rock layers on each side of the canyon gateway can hold an earthen fill dam.
A primary tool for geotechnical investigation — an effort to evaluate the condition of the bedrock — has been down-hole testing in which water is forced into the rock below and the rate of water taken into the rock is measured. Dinsick said 92% of the tests showed the rock was impermeable.
“Everything came up with passing grades,” he said. “It’s a blue-ribbon site for putting a water storage facility.”
Some permeable rock identified
According to progress reports, the investigations by Terra/GeoPentech of San Francisco have identified permeable conglomerate material that is sometimes a challenge for dam builders. Permeable means it allows water to pass through.
A report on the July 2024 meeting of the project’s Technical Review Board summed it up: “The investigation revealed that various foundation materials are relatively impermeable. However, the data has also shown that water takes are substantial at the contact of a fanglomerate sandstone and conglomerate. Therefore, special attention will be required for grouting and surface treatment of these zones.”
Dinsick said in a telephone interview that the builders will use grouting to seal permeable areas in the foundation rock in order to prevent seepage. “We have a good idea of what needs to be treated. All of the rock we tested was very tight. We didn’t find evidence of active faulting,” he said.
It turns out “fanglomerate” is a key term in discussing dam building on the eastern flank of the Diablo Range. Dinsick said the rock type is continuous on the ridge line along I-5 between Los Banos and Tracy.
The sedimentary rock is resistant to weathering, he said. It was originally sandy, gravelly material deposited in alluvial fans but was changed over time by calcium carbonate and groundwater. The testing recovered 5-foot pieces of the well-cemented layers, which look like a cross-section of aggregate concrete, Dinsick said.
He said the layers of permeable rock discovered are sandwiched between impermeable rock below and above. The project’s Technical Review Board, consisting of five geotechnical and consulting engineers, has suggested the design team provide specifications for the grouting in advance and that the grouting contractor and its personnel have at least five years experience on reservoir projects.
The federal environmental study released this month for public comment said additional geotechnical investigation may be required to support the Del Puerto dam’s design and may require a separate environmental analysis.
Garry Hayes, a geology professor at Modesto Junior College, gave his opinion on the geology at the dam site.
“The conglomerate and sandstone are probably stable in terms of the rock’s hardness and stability to serve as an anchor of the dam,” Hayes said by email, though he would defer to dam engineers. “I suspect the dam footprint will also include shale and claystone, and they are not particularly stable when soaked in water.”
In August, a water district dropped out of building the Pacheco Reservoir project in southern Santa Clara County due to cost overruns and unstable geology. That reservoir site also is in the Diablo Range.
Hayes has also raised concern about instability of slopes and active landslides in the Del Puerto reservoir inundation zone. Environmental studies have failed to address the possibilities of large-scale slope failures when the dam is filled, his email said.
“I am worried about what happens when a slide that formed under arid conditions is subsequently inundated beneath a hundred feet or more of water,” Hayes wrote. “The presence of water tends to reactivate old slides and slumps or can cause new ones to develop.”
A report on a November 2024 meeting of the Technical Review Board for Del Puerto Reservoir claimed the landslide hazard will not be significant. Landslide waves that could cause damage are unlikely because the slides are slow-moving, the report said. An analysis showed that a landslide would produce 5- to 9-foot high waves at the source and would be smaller upon arrival at the main dam or a saddle dam and would not overtop the dam crests, the report said.
Del Puerto invested in other projects
The federal environmental study also considered an alternative site for the reservoir at Ingram Creek near Westley, which would have the same operational approach — pulling water from the Delta-Mendota Canal and pumping it out when it’s needed in dry years. Critics note that the Del Puerto Water District has a stake in the project to raise San Luis Reservoir near Santa Nella and also has an Orestimba Creek groundwater recharge project. It’s suggested that Patterson-area farmers won’t see many benefits from Del Puerto Reservoir.
Hansen, Del Puerto’s general manager, said the district’s share of the San Luis expansion is 5,000 acre feet, which does not solve the district’s storage problem. “It’s not enough to keep the district viable in years of low allocation,” she said. “We had four straight years with zero allocations.”
As for funding for Del Puerto Reservoir, the partners plan to finance much of the $1.2 billion in costs, have a 25% cost-share from the federal government, a small state grant and may consider the state revolving loan program, Hansen said.
Funez said the canyon is worth saving and he will keep fighting in an era giving less priority to dam-building in California. He talked about the grinding stones and other cultural artifacts dating to when it was territory of the Yokuts tribe.
“The native American cultural sites are all over there,” said Funez, who plans to comment on the federal EIS. “We made contact with an elder at the Tule River Reservation and plan on sending some of these findings to see if they are aware.”
The public can comment on the federal environmental study through Jan. 12, either by email or sending a comment to Allison Jacobson, Bureau of Reclamation, 2800 Cottage Way, Sacramento 95825.
This story was originally published December 22, 2025 at 6:00 AM.
Join the Conversation
Anyone can read the comments, but you must be a subscriber or logged in with a registered account to contribute. If you do not have a registered account, you can sign up for one below (it's free).
Conversations are opinions of our readers and are subject to the Community Guidelines.
All Comments
Active Conversations
The following is a list of the most commented articles in the last 7 days.
A trending article titled "Can Steyer drop California electricity rates 25%? Don’t hold your breath | Opinion" with 1 comment.
Can Steyer drop California electricity rates 25%? Don’t hold your breath | Opinion
1
A trending article titled "Accusations of corruption, theft flung during Modesto water board meeting" with 13 comments.
Accusations of corruption, theft flung during Modesto water board meeting
13