Anderson Dam

Valley Water Board of Directors approve next step in fixing Anderson Dam

By John L. Varela, representative for District 1

Valley Water has many critical projects in various stages of development, including flood protection projects, infrastructure improvements and work to protect our environment.

But our top priority remains an effort to retrofit and strengthen Anderson Dam, home to Santa Clara County’s largest reservoir, so it can safely withstand a strong earthquake.

Recently, our Board of Directors took a step toward achieving this goal by approving the preliminary project description to empty Anderson Reservoir beginning Oct. 1 to the lowest practicable level possible. Draining the reservoir is the first in a series of activities aimed at seismically retrofitting Anderson Dam to modern-day safety levels.

Our project team is also preparing to begin construction of a larger low-level outlet tunnel in early 2021 which will allow us to better manage the reservoir level, and as a result, minimize the risk to public safety. We anticipate construction will begin in early 2021 and last 18 to 24 months.

Once the outlet tunnel is built, Valley Water will begin work on retrofitting the dam itself. That project is currently estimated to begin in 2024 and will last about seven years.

Our efforts to drain the reservoir and expeditiously build an outlet tunnel is in response to directives Valley Water received on Feb. 20 from the Federal Energy Regulatory Commission (FERC), the agency that has oversight of Anderson Dam. FERC also directed Valley Water to maintain Anderson Reservoir at about 35 percent of capacity until we begin to empty it this fall and continue to work swiftly to design and secure the necessary permits for the seismic retrofit.

Valley Water has complied with all the directives since receiving the order from FERC, and we remain on target to comply with all the requirements.

Valley Water will drain Anderson Reservoir in a safe manner, doing this gradually so as not to harm the creek channel, our water supply, or the environment. Valley Water plans to put the water currently in Anderson Reservoir to beneficial use by sending usable water to our three drinking water treatment plants and recharging the groundwater basins.

Other measures that will be taking place as we build the low-level outlet include downstream flood protection mitigation to Coyote Creek. We want the creek to be able to handle greater flows from the reservoir once the larger outlet tunnel is complete.

We remain optimistic that a bill making its way through the State Legislature will soon pass and make changes in law that will help facilitate the expedited and expert construction of Anderson Dam. The Expedited Dam Safety for Silicon Valley Act, a bill sponsored by Valley Water and authored by Assembly Member Robert Rivas (D-Hollister), was recently passed by the State Assembly with unanimous, bipartisan support. The bill is headed to the California Senate for further consideration.

One question that I’ve heard on multiple occasions is why is it taking so long to fix Anderson?

Since Valley Water first began efforts to strengthen the dam in 2012, the scope of the project has significantly expanded. Valley Water learned in 2016 that the dam itself needed a greater retrofit than originally thought, and the project doubled in size. In 2017, the state Division of Safety and Dams changed the design criteria for spillways, which required the existing spillway at Anderson be completely replaced as opposed to repaired.

Despite these hurdles, we are making progress, and I am optimistic that we have the momentum needed to get this project completed.


  1. Has any consideration been given to building a second dam; located about 500 yards East of the existing dam? In geographical terms the location is a “strait”, a narrow body connecting two larger bodies. The distance between the two points of land is half that of the existing dam; and a much higher dam could be built. And keep both dams; one higher than the other.


    1. Value engineering studies for the Anderson Dam Seismic Retrofit Dam Project have been performed that evaluated several dam site locations. The site referenced in your question had been evaluated in the past and was determined to be infeasible. This site location was found to be impracticable due to several site constraints and constructability issues that include:
      * Increased size and length of tunnel required for a new outlet works and diversion during construction
      * Lack of a suitable alignment for a new emergency spillway
      * Large size of cellular cofferdam that would be required for construction, and
      * Increased duration of construction due to the need to install a grout curtain under the core of the dam.


  2. This is BS. The reason all this started was so that land speculators could get approval to make millions building homes below the dam, which wouldn’t have been easily accomplished without this legislation. This project is never going to be finished, not until every penny is squeezed out of the taxpayers or we all die of thirst. Have fun ruining the rest of this valley, it’s only been a hundred years since it was delightful to live here. Don’t even get me started on the history of the exploitation of the native inhabitants of what was once the most beautiful land on the planet.


  3. I wonder how much those new multi-million dollar home owners below the dam are paying for flood insurance, if they can even get it. Are they “self insured”? My dad worked on the Oroville Dam. Both Oroville and Anderson dams are earth filled dams. Dad said that type dam is not safe, especially from earth quakes and liquefaction. That’s why they evacuated all of Northern Calif a couple years ago when the lake filled to capacity. Best bet for flood insurance? Genesis 5:32-10:1. An ark in the back yard. “It’s not nice to fool Mother Nature”. The same same situation exists with other County reservoirs; eg: Lexington …. The water district is putting profits ahead of safety.


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