A 2012 independent engineering study identified Calero Dam as a seismic risk due to the presence of alluvium – gravel and sand from the underlying creek bed – under the downstream dam embankment. That material could liquefy during a major earthquake on a nearby fault and cause the dam to deform significantly, risking an uncontrolled release of reservoir water. In response, the state Division of Safety and Dams (DSOD) imposed storage restrictions for the reservoir of 19-feet below the spillway crest, keeping water levels lower than normal to prevent topping in the event of a major earthquake until the water district assesses and conducts corrective action to restore the dam’s full integrity.
Calero Dam Background
Calero Dam was built in 1935. It measures 98 feet tall and 840 feet long and can store 10,000 acre-feet of drinking water. A 2012 Study determined that the dam embankment would fail during an earthquake of 7.9 or higher magnitude. Valley Water is completing its design phase with improvements to address this finding effectively.
Calero Dam Objectives
Alluvium, a material that consists of gravel and sand, exists at Calero Dam under the downstream embankment and could liquify during a major earthquake. In addition to potentially causing the dam to deform, there's a risk of uncontrolled release of reservoir water. Planned dam improvements include:
- Earthquake Safety: reconstructing and thickening the dam’s embankment;
- System Upgrades: constructing a new outlet system (releases water from the reservoir);
- Flood Protection: constructing a new spillway to current design standards and,
- Historic Structures Relocation: Relocating a historic house from Bailey Ranch and breaching Fellows Dike; and
- Safety Enhancements: incorporate other measures to address seismic and safety issues.
Project Update and Timeline
Initially, Calero Dam repairs would have required fully draining the reservoir, but to maintain water supply reliability, Valley Water is now developing a strategy to perform significant portions of the retrofit without reservoir drawdown, allowing for faster project completion. A new design consultant evaluating construction phasing, prioritizing embankment and spillway upgrades before outlet work improvements, with 50% design for the first phase anticipated by summer 2025. Alongside this, environmental assessment is being reactivated, with the Draft Environmental Impact Report expected for public review in late 2025. The remaining improvements will be designed under a phase 2 timeline.
January Public Meeting
The presentation from the January 23 public meeting is available to download.
The Calero Dam Seismic Retrofit Project will:
- Stabilize dam embankments
- Replace and modernize the outlet works
- Replace and modernize the spillway to increase freeboard
- Breach Fellow’s Dike, an older and smaller dam located on the southern-most section of the reservoir that is severely deteriorated.
The Santa Clara Valley Water District built the Calero Dam during the Great Depression, completing it in 1935 after acquiring land as the Santa Clara Valley Water Conservation District. The dam and reservoir is one of the six original reservoirs approved for construction by voters in May 1934.
The reservoir has a surface area of 349 acres and is a popular venue for boating, water skiing and jet skiing. The dam is critical to the district's water storage and management, capturing runoff from the nearby foothills and transfers Almaden Reservoir.