Calero Reservoir sits in the hills of south San Jose, below the Almaden Valley. Valley Water says the district built Calero Dam during the Great Depression, finishing it in 1935, and that both the dam and the reservoir are on Calero Creek.
What the reservoir does
Valley Water says Calero sends water directly to drinking water treatment plants. It also captures and stores winter runoff to recharge groundwater basins, helps store water from the nearby Almaden Reservoir watershed and accepts imported water. The district lists it as 2.2 miles long, with 9,934 acre-feet of storage and 349 acres of surface area.
Wikipedia describes Calero Dam as an earthen dam 90 feet high and 840 feet long, and calls the reservoir the fourth largest owned by the water district.
Why the water is low
According to Wikipedia, a 2012 independent engineering study found alluvium deposits beneath the dam that could liquefy in an earthquake. Since then the reservoir has been kept 19 feet below the spillway. Wikipedia says retrofit construction can begin only after the Anderson Dam retrofit is finished in 2031. Valley Water separately notes that planned improvements at nearby Almaden Dam are on hold until it finishes work at the Anderson, Calero and Guadalupe dams.
Visiting
Wikipedia says a 4,471-acre county park surrounds the reservoir, with catch-and-release fishing, picnicking, hiking and horseback riding. Swimming is prohibited, but boating, water-skiing and jet-skiing are allowed.
Anglers should know that the California Office of Environmental Health Hazard Assessment has issued a Do Not Eat warning for fish caught there because of elevated mercury levels, according to Wikipedia.