When The Change occurred on March 17, 1998, electricity and engines (both internal and external combustion types) immediately stopped working. Los Angeles County's population of nine and a half million people were dependent on water piped from hundreds of miles away and food transported on trains, trucks, and ships that no longer worked. The natural carrying capacity was about a quarter million people so many tried to flee while others hunkered down trying to ride out what they hoped was a temporary event. Neither helped and so the Great Southern California Dieoff occurred killing about 98% of the population.
The San Fernando Valley was badly hit with many dying. The survivors initially scavenged what they could from abandoned buildings but eventually began subsistence farming in parks and road verges. There was a need for organization so a pseudo-medieval society arose. Bruce Delgado formed up the Chatsworth Lancers and managed to dominate the west end of The Valley. He gained control of the water system and organized work crews to expand the land available for farming by breaking up old parking lots and covering collapsed buildings with soil. So long as there wasn't drought, the population stabilized at about 20,000 to 30,000.
Other more isolated and less crowded areas such as Topanga did better. The population was used to doing without and water in streams ran year round, most years. Most of the people survived by scavenging in The Valley before it became organized and by garden farming and hunting. As such, it was more loosely organized with authority vested with a ruling council (the "Brains") mostly to deal with the outside world.