Aerial view of Patterson Pass Water Treatment Plant.

Water Reliability

Focusing on the long-term needs for the Tri-Valley

How does water get to Zone 7?

Zone 7 provides wholesale treated water supply to four retailers, who then provide water directly to homes and businesses. Zone 7 also provides untreated water to agricultural users, golf courses, and other users. In an average year, Zone 7’s sources of water supplies are as follows:

Imported surface water:

Approximately 90% of  Zone 7’s water supply is imported through the State Water Project. Our water begins its journey in the Sierra Nevada where rainfall and melted snow make their way into Lake Oroville, one of the largest reservoirs in the state, where it is stored until needed. After being released from Lake Oroville, water flows along Feather River, into the Sacramento River and to the California Delta. From there, water is pumped into the California Aqueduct and then pumped into the South Bay Aqueduct, where it travels another 10 miles to the Tri-Valley.

Local water:

Our local water supply consists of rainfall captured in a local reservoir, Lake Del Valle, which is also part of the State Water Project system. This makes up approximately 10% of Zone 7’s water supply in a typical year. The Livermore Valley Groundwater Basin, our local groundwater basin, acts as our water “savings account” because we store excess surface water supplies in the groundwater basin during wet years for later use.

What happens during dry years? 

California rainfall and snow can vary tremendously from one year to the next. To deal with this variability in water supplies, Zone 7 stores extra water during wetter years for use during dry years such as 2020-2021. Zone 7 uses both aboveground or surface water reservoirs and underground reservoirs called aquifers or groundwater basins to store water. Currently, Zone 7 is able to store water in the local groundwater basin, in Lake Del Valle, in San Luis Reservoir through the State Water Project, and the Kern Storage and Recovery Programs. Water from the local groundwater basin is pumped directly by Zone 7. Water stored in Lake Del Valle is delivered via the South Bay Aqueduct. The rest of Zone 7’s stored water is delivered as surface water through the Delta and South Bay Aqueduct through a system of ‘water exchanges’. In a dry year, stored water can make up the majority of the water delivered by Zone 7 to its customers!

Planning for Water Supply Reliability

2022 Water Supply Evaluation Update

Every few years, Zone 7 performs an update to the Water Supply Evaluation, a document that presents analysis of Zone 7’s future water supply and storage alternatives needed to maintain the reliability of the Tri-Valley’s water supply system. The 2022 Water Supply Evaluation Update began this year with the development of a new water supply risk model. The new risk model is catered to water supply planning and utilizes refined data. The risk model will be used to evaluate potential water supply reliability projects for Zone 7 and findings will be captured in the 2022 Water Supply Evaluation Update, which will help inform the Zone 7 Board’s future investment decisions.

Water Reliability Project Updates

To maintain the reliability of Zone 7’s water supply system into the future, Zone 7 is pursuing sources of new water supplies and additional storage, as well as improvements to water delivery facilities.

Los Vaqueros Reservoir Expansion

up to 10,000 acre-feet of water storage

Zone 7 continued to invest in the Los Vaqueros Reservoir Expansion Project. The project seeks to expand the existing reservoir to a capacity of 275,000 acre-feet. This project is located in Contra Costa County and will serve as an upstream storage facility that will increase water supply reliability in the Bay Area. This project will also build a new pipeline, the Transfer-Bethany Pipeline, which would provide a new way for Zone 7 to access water supplies. This year, the Los Vaqueros Reservoir JPA was formed by project participants to advance the project. The project also made progress in environmental planning, design and engineering, and securing funding. If this project were online during a critically dry year like 2022, Zone 7 would have more operational flexibility and more water available from storage to supplement low supplies.

Sites Reservoir

Average reservoir yield of 10,000 acre-feet per year

Water Transfers

10,000 acre-feet purchased this fiscal year.

To bolster water supplies during this dry year, Zone 7 purchased about 10,000 acre-feet of water. This water was purchased from another State Water Project contractor, Mojave Water Agency, and from long-standing programs such as the Dry Year Transfer Program and the Yuba Accord.

Water Quality

Flood Protection