Hi Kate! Lake Casitas is located on the eastern edge of the Santa Ynez Mountains (the east-west trending mountain range just north of Santa Barbara). To get to Los Angeles from Santa Barbara, you pass through three mountain chains: the Santa Ynez Mountains, the Santa Susana Mountains, and the Santa Monica Mountains. There are two main mountain ranges in the LA area: the Transverse Ranges run east-west (and they're one of only two in the US that do so!), and the Peninsular Ranges run approximately north-south.

I've attached a screenshot of Google Maps that I've annotated for you & labeled the different mountain chains in the area you're interested in. Have a great day!