Water being released from Anderson Dam to maintain 3.7% storage level          

In February 2020, Valley Water received an order from the Federal Energy Regulatory Commission (FERC) to lower Anderson Reservoir to the lowest level possible, around 3.7% of the reservoir’s total storage capacity. This FERC order was given to reduce the risk to the public should Anderson Dam fail during a major earthquake while Valley Water builds a new, larger outlet tunnel. Consequently, Valley Water began draining Anderson shortly after and has maintained water levels at 3.7% since December 2020.

The recent series of atmospheric river storms – described by the Mercury News as the second wettest 21-day stretch in the Bay Area since 1849 – filled many of Valley Water’s 10 reservoirs. Rainfall was significant across Santa Clara County, including watersheds around Anderson Dam in Morgan Hill, where between 13 and 17 inches of rain fell.

These storms resulted in significant rainfall runoff into Anderson Reservoir. The amount of water that entered the reservoir exceeded the amount that could be released through the existing outlet pipe, which was fully open during the storms. Between Dec. 31 and Jan. 18, the amount of water stored in Anderson increased from 3.9% to about 45% of capacity. It will take Valley Water about two months to bring Anderson Reservoir back down to 3.7% capacity without additional rainfall.

The existing outlet pipe at Anderson Dam can release up to 1,000-acre feet (or 325 million gallons) of water daily. To view water levels in Anderson Reservoir and other nine reservoirs, as well as creeks and streams, visit Valley Water’s ALERT (Automated Local Evaluation in Real Time) system.

Not all the water released from Anderson Reservoir flows to the San Francisco Bay. In fact, some of that water flows from Anderson Reservoir along Coyote Creek, which percolates through the creek bed within the Coyote Valley and southern San Jose to refill or “recharge” the aquifer. Allowing this water to flow from Anderson Reservoir along Coyote Creek also has an environmental benefit for habitats within the creek.

The steady releases of water from Anderson Reservoir into Coyote Creek are not expected to result in flooding.

Valley Water is continuing our work on projects to reduce the risk of flooding in neighborhoods along Coyote Creek in San José. The Coyote Creek Flood Protection Project and the Coyote Creek Flood Management Measures Project extend for nine miles in historically flood-prone areas and seek to protect residential, commercial, industrial areas and major roads and highways from floods.

In February, Valley Water will remove trees and vegetation in preparation for the start of construction on the Flood Management Measures Project.

Valley Water is also making progress on our work at Anderson Dam, where we are building a 1,700-foot-long tunnel, up to 24 feet in diameter, on the left side of the dam as you look toward the reservoir. The recent rain and storage levels with Anderson Reservoir are not slowing down work on the tunnel.

Once completed, the new outlet tunnel will increase the amount of water that can be released from Anderson by five times, allowing Valley Water greater control over water levels in the reservoir. We anticipate work on the outlet tunnel will take another two years to complete.

Valley Water will begin retrofitting the dam embankment and spillway once the tunnel is complete.  That effort, known as the Anderson Dam Seismic Retrofit Project, is estimated to start in late 2025 and will last about seven years.

For more project information, including meeting materials, frequently asked questions, reports and other project-related documents, please visit the project web page at http://Delivr.com/2uanu.


  1. And so we will be in excessive drought again from June… Has anyone done a cost/benefit analysis of the real risk of a serious earthquake and its impact on flooding vs the extra cost put on all citizens from excessive water rates from, what seems to me based on this, a manufactured excessive drought crisis?


    1. Let me see if I understand your question… are you asking if anyone has analyzed the cost of thousands of homes and lives being lost due to a catastrophic failure of Andersen Dam, which HAS, in fact, been deemed unsafe by MULTIPLE investigations.. versus the benefit of you paying a little bit less for your water?
      If you want to have access to that water reserve, a new/modern dam is necessary. That will cost about a billion dollars.. are you willing to be taxed to pay for that construction? How about a water usage levy?
      Oh.. by the way.. it would have cost 1/4 of the current cost 20 years ago when they uncovered the fatal design flaw, but fiscal conservatives decided to ignore it and just roll the dice. Are you one of those? …are you, Bart?


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