Most of the West and East slope watersheds are already heavily dammed. But the Clean Water Act regs require so much to be left to flow into the Delta. And then there is the Peripheral Canal...
California's best bet is desalination/nuclear plants. But it will never happen until all the Greens turn brown.
Some people happen to like flat water recreation, clean inexpensive energy and lots of fresh water that has innumerable uses and re-uses, rather than just letting it run back out to sea as usless stuff.
Which do you think the greenies would allow first... Nuclear desalinization or a few more on stream reservoirs?