However, the lack of sufficient reservoirs is indeed a real problem. The population and it's water needs have greatly eclipsed the state's storage capabilities, which creates a negative feedback cycle during dryer seasons/years.
I'm sure people who are more knowledgeable about the problem are trying to work out solutions, and there is probably a straightforward answer to why we don't have a working solution already?
https://www.mercurynews.com/2014/08/31/california-drought-wh...
Here is a more recent article talking about the proposed Sites Reservoir the other article mentions.
https://www.ktvu.com/news/a-new-mega-reservoir-in-final-plan...
As an example, the LA River is a giant concrete channel that rushes water into the Pacific. There is some support for restoring it into a more absorbent wetland state but it will take decades and doesn’t cover the whole river.
As a more farcical example, the recent storms have downed a tree across the Caltrain commuter rail line. They are slow on removing it because the jurisdiction the tree fell in consider it a historic tree despite the fact that it is eucalyptus, a species invasive to California.
That's not to say environmental reasons aren't good reasons (with a certain balance of course).
However, in California, this has become the "go-to" excuse for blocking most new public-works projects, often tying up projects in decades of litigation and studies... which typically means the project is dead before it even starts. Californian's have become skeptical of these weaponized "studies" as a result.