Valley Water evaluating five dam alternatives for the proposed expansion of Pacheco Reservoir

In the face of climate change and severe weather, there is a risk of more prolonged droughts in California. Despite recent storms in March, Santa Clara County is now in a drought and it is unknown how severe it will get.

Valley Water remains focused on preparing for future dry and wet years through a variety of projects and programs, including the proposed expansion of Pacheco Reservoir in southern Santa Clara County. The project would increase the reservoir’s capacity from 5,500 acre-feet to up to 140,000 acre-feet, enough water to supply up to 1.4 million residents for one year.

The project is a collaboration between Valley Water, the Pacheco Pass Water District and the San Benito County Water District. Pacheco Reservoir is located 60 miles southeast of San Jose and sits to the north of Highway 152. The existing dam is about 100 feet high and is owned and operated by the Pacheco Pass Water District.

In February 2021, Valley Water held two virtual public scoping meetings and detailed five dam alternatives being evaluated for the Pacheco Reservoir Expansion Project as part of the California Environmental Quality Act process. Valley Water is also evaluating an option of no expansion.

Variations of the alternatives include two different locations, two dam types and two reservoir sizes. The cost estimates for the alternatives range from $2.1 billion to $2.5 billion.

The two locations are described as the upstream site and downstream site. The upstream site, located about 1.7 miles from Highway 152, would allow for a greater amount of channel restoration between the new dam and the existing dam. This would create new habitat for the threatened South-Central California Coast Steelhead population and other species.

The downstream site is located about one mile from Highway 152 and is closer to the existing dam.

The reservoir sizes being considered include a 140,000-acre foot reservoir located at both sites, and a 96,000-acre-foot dam located at the upstream site.

Two different dam types being considered include a hardfill and earth filled dam, which are both designed to withstand a large earthquake.

A hardfill dam is a newer technology that uses rock materials mixed with cement and compacted together. The strength of this material allows for steeper slopes on the dam faces and a smaller footprint. The spillway and inlet/outlet pipelines can be built into the hardfill dam structure, allowing reservoir overflows to spill over the body of the dam through a channel.

An earth fill dam is similar to Valley Water’s existing dams and would require a separate spillway and inlet/outlet pipelines.

All dam alternatives include:

  • A new dam and expanded reservoir upstream of the existing dam.
  • A connection to the existing Pacheco conduit that will allow for water from San Luis Reservoir to be pumped and stored in the existing Pacheco Reservoir. Pacheco Reservoir is also located about eight miles west of San Luis Reservoir, which is jointly owned and operated by the U.S. Dept. of Reclamation and the California Dept. of Water Resources.
  • The removal of the existing dam and restoring the channel between the old and new dams.
  • Making roadway improvements from Highway 152 to the dam site.
  • Building new powerlines.
  • Activities related to construction of the dam.

Video recordings of the virtual scoping meetings held in February can be viewed at

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