A 2011 independent engineering study has labeled the Calero and Guadalupe dams a seismic risk due to the presence of alluvium - gravel and sand from the underlying creek bed - under the downstream dam embankments. That material could liquefy during a major earthquake on a nearby fault and cause the dams to deform significantly, risking an uncontrolled release of reservoir water.
Each dam is about 80 years old and is critical to the district's water storage and management. In response to the study, the state Division of Safety and Dams (DSOD) imposed storage restrictions for the reservoirs of 20-feet below the dam crest, keeping water levels lower than normal to prevent over spilling until the water district assesses and conducts corrective action to restore the dam's full integrity.
The water district's Seismic Retrofit Project will reinforce the stability of the dams to put them in better shape to withstand a large quake. At Calero Reservoir, the project will also improve the Fellows Dike, an older and smaller earthen dam on the reservoir's southern most section. The DSOD has deemed that dam as deteriorated and is restricting the district from impounding water against it. The restriction resulted in a 2,000-acre feet - about one-fifth of reservoir capacity - reduction of water stoage capacity in the reservoir.