Expanding Pacheco Reservoir will ensure a more reliable supply of safe, clean drinking water for our region

Many Santa Clara County residents know how unpredictable our water supply can be. The last decade brought the Golden State five consecutive years of drought, followed by one of the wettest winters on record.

In the face of climate change, Valley Water remains focused on preparing for future wet and dry years to ensure Santa Clara County’s 2 million residents have a reliable supply of safe, clean drinking water.

We are currently developing one of our biggest projects in decades, the proposed expansion of Pacheco Reservoir in southern Santa Clara County. The Pacheco Reservoir Expansion Project is a strategic and forward-thinking, long-term investment toward ensuring a more reliable water supply for future generations.

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 proposed project will boost Pacheco Reservoir’s operational capacity from 5,500 acre-feet to up to 140,000 acre-feet, ensuring a more reliable supply of safe, clean drinking water.

In Santa Clara and San Benito counties, about 40% of our water supply is imported from the Sacramento-San Joaquin Delta, where water infrastructure is aging and vulnerable to earthquakes, levee failure and extreme weather conditions. In an emergency, we could lose that water for up to 18 months. The expanded Pacheco Reservoir could provide a year’s supply of water for up to 1.4 million people in an emergency.

In Santa Clara County, nearly half of all our water is pumped from underground basins. If we pump too much from our underground aquifers, the ground could sink, potentially causing long-term harmful impacts to roads, bridges and pipelines. Expanding Pacheco Reservoir would double the amount of water we can store above ground, making us less reliant on our groundwater supply during droughts.

The expanded Pacheco reservoir would enhance conditions for the threatened South-Central California Coast Steelhead population by providing suitable water flow and temperature to support the migration and survival of these fish.

In July 2018, Valley Water, the Pacheco Pass Water District and the San Benito County Water District collaborated to secure $484.5 million in funding from California’s Proposition 1 Water Quality, Supply and Infrastructure Improvement Act of 2014. The Act, passed by California voters, provides for $7.5 billion in general obligation bonds, including $2.7 billion for investments in surface and groundwater storage projects. The $484.5 million in funding was conditionally approved and accounts for more than one-third of the estimated $1.3 billion project cost.

Valley Water is also seeking federal funding and exploring other avenues to reduce the cost of the project to ratepayers.

Construction to expand Pacheco Reservoir is anticipated to begin in 2024. Preliminary estimates indicate construction would take at least five years.

To receive the latest information about the project or upcoming meetings, please sign up on the project’s webpage at valleywater.org/pachecoexpansion.


  1. While the expanded reservoir can hold the quantity you state, will there be enough ever available to fill it a decade from now when construction is complete? I believe the majority of imported water now accessible from the Delta originates from snowmelt in the Sierra’s and nearby. Due to current supply versus demand curtailments of Delta imported waters during drought periods and even during normal rainfall years, what do studies show is the statistical probability of enough water being available to fill the new reservoir in addition to existing storage reservoirs under current conditions? A study last week by Stanford News
    points out that expected changes in climate in the near future will involve less snowfall over shorter periods and more extreme storm weather conditions causing loss of water to the sea through unexpectedly large and frequent flooding. My sole purpose in calling this out is to assure ratepayers that they won’t have to pay for a beautiful oversized dry edifice and then have to pay an extensive addition to control and store excess flood waters before they flow to the ocean (once adequate studies can provide accurate impact and design analysis).


    1. Thanks for the question.
      You are correct that the climate change will change the source and timing of water we import. It is predicted that climate change will result in more spring runoff and less snowpack (for early to mid-summer supplies). Climate change models suggest more water will be available but at much shorter windows to pump from the Delta. This is exactly the scenario a reservoir such as Pacheco is needed. We can fill the reservoir at times when excess flow at Delta is available to import, store it, and deliver throughout summer and fall. When we applied for and were conditionally approved for Proposition 1 funds for the Project, we looked at 2030 and 2070 future conditions with climate change to make sure the reservoir is not only viable but stands the changes expected in the future.


  2. What are the plans to supply Public water to the areas of the county not presently served such as San Martin , Gilroy etc.


    1. The cities of Morgan Hill and Gilroy, and unincorporated areas like San Martin depend almost entirely on groundwater for their water supply. Valley Water does not directly supply groundwater to residents or businesses, but actively manages groundwater beneath the valley floor to ensure there is enough water for municipal, industrial, domestic, and agricultural users as well as groundwater-dependent ecosystems. Imported water from the Sacramento-San Joaquin Delta watershed is an important water source for South County, especially in dry years when it can provide more than half the water used to replenish groundwater. Some of the water stored in an expanded Pacheco Reservoir could be used to help replenish the groundwater supplies in South County.


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