A nuclear apocalypse happens and what not. 500 years later the region of Southern California has devolved into primitive and backwards tribes that are often at war with each other.
In this story, my main character helps one tribe defeat two others in a war to control the water resources in the area.
The thing is, it would make more sense for the clans to just relocate somewhere else instead of staying. My question is, what would be a logical reason for the tribes to not leave the area?