About that dam, Wiki says: https://en.wikipedia.org/wiki/O%27Shaughnessy_Dam_(Californi...

... that it produces about 976 GWh per year. A few different sources say that SF uses about 5-6 TWh per year. So that is slightly less than 20% of total power needs. Still, it is a good start.

SF has other power generation besides Hetch Hetchy.