Central valley all used to be marshlands/lakes/extensions of the bay. The coastal regions of CA are hills. CA is basically a giant bowl (which flattens out towards the south).
That should be pretty feasible to wall off though.
Elevation of Sacramento is 30 feet....that's lower than the bay bridge.
Right, totally not what one would expect. But based on the geography of California, it's very vulnerable if the water levels were to go up significantly. There would essentially be a second Bay cutting deeper into the state, only it would probably be a toxic mess a few feet deep with acres of asphalt and buildings barely submerged.
In all likelihood, Sacramento and SF bay would be fine because it would be inexpensive to levee.