The most prime spots for reservoirs have actually all been taken, and: "Reservoirs only store water, they cannot create it".

https://californiawaterblog.com/2011/09/13/water-storage-in-...

So pretty clear the environmental damage caused by a new one is not worth it. Rather, conservation of our groundwater and reducing wasteful agricultural practices in the south central valley would be our best bet at ensuring drought-time water security.