I believe you are incorrect. The Crystal Springs Lakes provide terminal storage for Hetch Hetchy water. They are immediately south of SF and hold a huge amount of water. I drove between them a couple of weeks ago, and they were nearly full.
Crystal Springs Reservoir is a man-made lake. The dam that holds the water back is pretty old, and the San Andreas Fault runs right through the reservoir. Downtown San Mateo is downstream from that dam.
What could go wrong?