After toiling away in the remote hills east of Interstate 680 on the Alameda-Santa Clara county line for seven years, hundreds of construction workers have finally finished the largest dam built in the Bay Area in 20 years. The 220-foot tall dam at Calaveras Reservoir — as high as the roadway on the Golden Gate Bridge soars above San Francisco Bay — replaces a dam of the same size, built in 1925.  State dam inspectors flagged the older dam in 2001 as at risk of collapse in a major earthquake on the nearby Calaveras Fault. If it had failed, state officials estimated it could have sent a 30-foot wall of water into Fremont and neighboring communities, potentially killing thousands of people.