Emily Green has written a great primer for Southern Californians on the Sacramento-San Joaquin Delta, the distant geography from which a big chunk of their water emerges:

[W]hat makes the Delta the Delta is water. After winter rain and snow, roughly half of California’s fresh water arrives in this quirkily engineered, mis-named place. Twenty five million Californians depend to some degree or another on freshwater from the Delta. Roughly a third of Southern California’s supplies originate here.

Emily is sneaky good. This is great geography and hydrology, but as she notes in the introduction, this is at heart about sharing water.