There's an entire book about this but the mile-high view is this:
1) California has never had enough water. Climatically it's Nevada with a coast.
2) California in general and Los Angeles in particular has been really good at stealing water. This is why the Owens Valley and the Coachella Valley are desert and the San Fernando And San Gabriel Valleys are not.
3) California is running out of water to steal.
Important to note: the Salton Sea is not a sea. It's a water spill. If there were no dams on California there might be one there now, or there might be one there in a hundred years, and it might be gone in another 150. But there are dams in California and as a consequence, what should look like the Bonneville Salt Flats instead has piers on it.
And a bunch of dead-enders that can't afford to leave. Mercy would be relocation assistance and mortgage buy-outs. Go pull up a Google Street View of Bombay Beach. Recognize that when the Google car cruised through there they probably blasted the A/C and locked the doors and didn't stop until Indio which is its own special hell.
Know what wiped out the Sumerians? Know what wiped out the Hohokam?