~99% of free flowing water in CA would evaporate or dump in a tiny number of rivers which are tapped for urban water. The only reason many of these watersheds appear separate is 100% of the water is removed long before it enters the ocean. The Kern River being a great example that used to dump into the San Joaquin River but none of that water flows into the ocean today.

https://en.wikipedia.org/wiki/List_of_rivers_of_California

PS: Wells generally tap flowing water even if it’s simply not obvious that’s what’s going on. Basically, the ground can only hold so much water and it normally absorbs excess from rain which eventually flows into streams etc.