San Jose is trying to 10x the size of the reservoirs but an Indian tribe is claiming that land is “sacred”. I guess it’s like the Navajo Indians claiming the moon as sacred and asking NASA to not land on it.

Alternatively, just add to the gigantic "reservoir" underground, i.e. the water table.

You could additionally store more water in the soil. Dig some basic water harvesting earthworks in the mountains, maybe plant a few select species and within a few years all the mountain streams will flow for much longer during a drought. As a bonus it will leave a lot more trees alive when the wildfires come through. Of course this is impossible in California, even if it didn't violate environmental laws (it does), a project like this would be sued and propositioned to oblivion.

Where I live, the population has grown since the 1970s, but the reservoir capacity has gone down (sedimentation). Doesn't seem politically feasible to build a reservoir anymore.

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.