A proposed reservoir project in Del Puerto Canyon, western Stanislaus County, has become the center of a debate involving seismic risks, environmental impacts, and concerns over wildlife and geology. The U.S. Bureau of Reclamation released an environmental impact statement (EIS) this month for public comment, renewing discussions about the safety and necessity of building a dam in the area just outside Patterson.
The Del Puerto Water District and San Joaquin River Exchange Contractors Water Authority plan to submit a construction application in 2026 for the $1.2 billion reservoir. If approved and funded, construction could begin as early as 2027. The reservoir would store up to 82,000 acre-feet of water across 4.5 miles of Del Puerto Canyon on the west side of Interstate 5.
Anthea Hansen, general manager of the Del Puerto Water District, stated: “The project is currently at 35% percent design for all facilities,” adding that coordination is ongoing with utilities regarding power line relocation and with Stanislaus County on a $113 million road realignment.
Supporters argue that the reservoir will help farmers by storing water during wet years for use during dry periods when Central Valley Project allocations are reduced. However, some local residents and conservation groups remain unconvinced that another reservoir will address California’s persistent water shortages or protect natural resources.
Elias Funez, volunteer with Save Del Puerto Canyon, said recreational opportunities like hiking and biking would be lost if the canyon is flooded by the new reservoir. “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.”
Environmental reviews have been contentious since at least 2019 when several organizations challenged an earlier state-required EIR in court. While a judge ruled largely in favor of the water districts—ordering further study only on road realignment—the Fifth District Court of Appeal later required additional review on terrestrial species along Del Puerto Creek downstream from the dam site.
Funez criticized the federal EIS as being derivative of previous reports while raising concerns about new seismic activity noted after recent earthquakes near Diablo Grande and Westley in September 2023. Although major earthquakes are considered unlikely locally, both state and federal studies reference nearby faults such as one located half a mile east of the main dam site.
Dam failure could potentially flood parts of Patterson by overtopping Interstate 5 and possibly affecting other infrastructure like the California Aqueduct; however, planners say seismic hazard analysis will guide engineering so that dam facilities provide “acceptable performance under a maximum credible earthquake,” according to their final EIR.
Recent sightings reported by KQED indicate that protected California condors have expanded their range into areas near Mount Diablo and Mount Oso north of Del Puerto Canyon—a development not addressed in current federal documents but acknowledged by wildlife experts as part of ongoing conservation efforts over recent decades.
Hansen said Thursday there was no record from their own environmental studies showing condor presence near the proposed site.
Geologists continue to study unique rock formations within Del Puerto Canyon which attract educational field trips from local colleges due to exposed sedimentary layers uplifted over millions of years through tectonic activity. Engineers have conducted three years’ worth of geotechnical testing on bedrock intended for dam foundations—finding most materials impermeable but identifying permeable conglomerate zones requiring special grouting treatment to prevent seepage.
Ron Stork from Friends of the River noted: “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… In California we have had failures along abutments before.”
Andrew Dinsick, deputy project manager for Del Puerto Reservoir stated: “Everything came up with passing grades… It’s a blue-ribbon site for putting a water storage facility.” He added they did not find evidence suggesting active faulting at tested locations but acknowledged some rock layers require targeted treatment prior to construction.
Some geologists—including Garry Hayes at Modesto Junior College—express caution about potential landslides within inundation zones once filled with water: “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 via email correspondence included with technical board reports indicating slow-moving slides should pose limited risk based on available analyses.
Alternative sites such as Ingram Creek were also reviewed but determined operationally similar; critics maintain local benefits may be limited given existing investments by participating agencies elsewhere including San Luis Reservoir expansion projects which do not fully meet long-term storage needs according to Hansen: “It’s not enough to keep the district viable in years of low allocation… We had four straight years with zero allocations.”
Funding sources include federal cost-sharing (25%), smaller state grants plus possible loans through revolving funds programs alongside direct financing from partners involved.
Funez emphasized cultural significance beyond recreation citing Native American artifacts throughout Del Puerto Canyon: “The native American cultural sites are all over there,” he said while noting plans to consult elders at Tule River Reservation regarding findings ahead of submitting formal comments before January 12th—the deadline set by Bureau officials for public feedback.
