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Valley Water working to expand reservoir near Highway 152 in Southern Santa Clara County

December 05, 2019
Pacheco Reservoir

Most people who drive along Highway 152 in southern Santa Clara County are unaware that a small reservoir sits less than one mile north of the roadway.

Unlike San Luis Reservoir, which sits adjacent to Highway 152 and is visible to passing motorists, the Pacheco Reservoir is secluded and mostly unknown.

A rendering of an expanded Pacheco Reservoir and dam in southern Santa Clara County.

But efforts are in place to expand Pacheco Reservoir into one of the largest reservoirs in this region. This collaboration between Valley Water, the San Benito County Water District and the Pacheco Pass Water District is a long-term investment geared toward securing a greater supply of safe, clean water in the face of climate change.

The proposed project will boost Pacheco Reservoir’s operational capacity from 5,500 acre-feet to up to 140,000 acre-feet, enough to supply 1.4 million people with safe, clean water for one year in an emergency. The expanded Pacheco Reservoir will also reduce the frequency and severity of water shortages during droughts, preserve groundwater and protect infrastructure, and enhance, protect and restore our environment.

Valley Water is presently conducting studies, investigations and surveys to design and evaluate potential environmental impacts and feasibility for the Pacheco Project.

The proposed expansion of Pacheco Reservoir includes the construction of a new, earthen dam a short distance upstream of the current dam. It also includes a pipeline that will allow Pacheco Reservoir to receive water from San Luis Reservoir.

Construction to expand Pacheco Reservoir is anticipated to begin in 2024.

Efforts to expand Pacheco Reservoir received a boost earlier this year when it was named as the preliminary preferred option to address an issue of algae growth at San Luis Reservoir. When algae growth occurs, it can impact the delivery of water from the San Luis Reservoir to Valley Water (which serves Santa Clara County) and the San Benito County Water District.

A rendering of an expanded Pacheco Reservoir and Dam, located about 15 miles west of San Luis Reservoir.

The Bureau of Reclamation and the California Department of Water Resources jointly own and operate the San Luis Reservoir to store water for the Central Valley Project (federal) and State Water Project. Valley Water and the San Benito County Water District receive some water yearly from the San Luis Reservoir, as do other water districts in California.

The low point problem at San Luis Reservoir sometimes occurs when warmer temperatures and low water levels foster algae growth, which makes the water unusable within Santa Clara and San Benito counties. San Luis Reservoir has reached the low-point level seven times since 1968; with climate change, this is expected to increase.

The effort to correct the algae issue at San Luis Reservoir, dubbed the San Luis Low Point Improvement Project, is being led by the U.S Bureau of Reclamation and Valley Water.

To address the issue, Reclamation and Valley Water studied dozens of possible alternatives and narrowed that list to five choices. Of the five, the Pacheco Reservoir Expansion Project emerged as the preliminary preferred option to address the low-point issue. Before a final decision can be made, more comprehensive evaluations and studies must be completed on the Pacheco project.

During potential low-point events, Valley Water will utilize the expanded Pacheco Reservoir to receive and store its supply of water from San Luis Reservoir. This will allow Valley Water and the San Benito County Water District to receive water from the state and federal supplies before algae growth becomes an issue.

By being named as the preliminarily preferred option, a path has been created for the Pacheco Reservoir Expansion Project to potentially receive federal funding to assist in the design and construction of this new dam.

The projects at Pacheco and San Luis reservoirs started independently, but they are converging on a common goal that achieves the objectives of both projects.

To receive the latest information about the project or upcoming meetings, please sign up on the project’s webpage at

Valley Water manages an integrated water resources system that includes the supply of clean, safe water, flood protection and stewardship of streams on behalf of Santa Clara County's 2 million residents. The district effectively manages 10 dams and surface water reservoirs, three water treatment plants, an advanced recycled water purification center, a state-of-the-art water quality laboratory, nearly 277 acres of groundwater recharge ponds and more than 275 miles of streams. We provide wholesale water and groundwater management services to local municipalities and private water retailers who deliver drinking water directly to homes and businesses in Santa Clara County.