What to expect when Valley Water begins lowering water levels in Anderson Reservoir 

Starting Oct. 1, Valley Water will begin lowering water levels in Anderson Reservoir in response to an order by the Federal Energy Regulatory Commission (FERC) to reduce the risk to the public should Anderson Dam fail during a major earthquake.

Lowering the reservoir is the first in a series of activities aimed at making Anderson Dam earthquake safe.

Valley Water will drain Anderson Reservoir at a rate of about 200-acre feet, or 65 million gallons a day. That’s equivalent to emptying 98 Olympic swimming pools every 24 hours. We will gradually reduce the water level to 3% capacity, which should take between three and six months. The timing is dependent on the amount of rain we receive this winter.

Once the water levels reach 3% of capacity, Valley Water will begin building a 1,700-foot-long tunnel, up to 24-feet in diameter, on the left side of the dam looking towards the reservoir. The new tunnel will increase the amount of water that can be released from Anderson by five times. We estimate it will take two to three years to complete the outlet tunnel.

Lowering the water levels of Anderson Reservoir and beginning construction on the outlet tunnel will have impacts on recreation activities, the surrounding community, and Coyote Creek.

Anderson is the largest reservoir in Santa Clara County and a popular recreation area.

To keep the public safe during construction, Valley Water is closing many recreation areas around Anderson Dam and Reservoir for several years, until the retrofit is complete. Starting Oct. 1, the following recreation areas will be closed:

• Toyon Group Picnic and Parking Areas; Serpentine Trail; Dam Crest
• Woodchoppers Flat
• Anderson Lake Park’s boating and fishing, boat and vehicle parking areas, and boat ramp; Coyote Road from the toe of the dam to the boat and vehicle parking areas; and Lakeview Trail is closed from the Anderson Launch Ramp parking lot trailhead to the westernmost junction with the Rancho Laguna Seca Trail.
• Fishing will be closed for the entire reservoir shoreline

As water levels in Anderson Reservoir are lowered, the bottom of the reservoir will become muddy. Due to the possibility of slipping into or getting stuck in the mud, please do not enter the reservoir.

Water released into Coyote Creek will result in higher flows than are normal for this time of year. Some stretches of the creek are narrower than others, and the adjacent Coyote Creek trail could see some ponding of water.

The reservoir will have a remaining pool of water roughly the same size as Stevens Creek Reservoir, and we expect most fish will be able to persist. Valley Water biologists will also perform a fish rescue for any fish trapped in small pools of water and place them in the remaining larger pool of water if feasible.

However, if lower water levels result in the deaths of fish there could be unpleasant odors around the reservoir.

Valley Water researched the best ways to minimize the impacts of the sediment. Sediments are a mix of sand, silt, and clay particles that can be carried out of the reservoir by flowing water. Sediments may end up settling in the creek at different locations.

Increased sediment may be seen in the reservoir releases or in areas of Coyote Creek due to the lowering of the reservoir. This sediment is anticipated to be washed out to the Bay in subsequent larger rainstorms.

We discussed impacts of lowering Anderson Reservoir at a virtual public meeting on Sept. 17. A recording of that meeting is available for viewing at https://www.valleywater.org/anderson-dam-project

After review and certification of an Environmental Impact Report currently being prepared, Valley Water Board consider approval of a long-term project, the Anderson Dam Seismic Retrofit Project (ADSRP). This proposed ADSRP calls for retrofitting the dam embankment and spillway once the tunnel is complete. ADSRP construction would start in 2024 and last about seven years.

The total cost of the project is estimated at $576.3 million. If Measure S passes in November, the Safe, Clean Water and Natural Flood Protection Program would provide a total of $54.1 million to the project. Valley Water is also looking to secure funding from state and federal partners. The rest of the project will be paid for by water rates, which has already been budgeted.

Valley Water’s priority is the health and well-being of our community. The proposed Anderson Dam Seismic Retrofit Project is one of Valley Water’s highest priorities to ensure we do our part to protect Santa Clara County residents.