You can put them here and comment from the perspective of a fictional world on the current battles over water that really are taking place in this region. That is high science fiction.
Key is that they must have need of the water: if these are a bunch of badass hunters they will not be so water oriented. They need to be farmers (or doing something else with the water) and they need to farm such that the water there is cannot support them all. Maybe they can be rice farmers - wildly impractical in this part of the world but doable. If this is absurdist fiction, take it one more step to impracticality: they are trout farmers and catch them via fly fishing.
Not implausible at all in modern SoCal. Putting this story on the desert Colorado River will bring home to your readers what it is you are commenting on. Except for the river, there is no surface fresh water for hundreds of miles in any direction. No-one who had any conception of what was out there would leave - unless they were following the river.
The gang concept from @Raditz_35 is excellent - culture of tough people in this part of the world will be influenced by gang culture and the SoCal gang tattoos are amazing.
other bonuses from locating story at actual site
You can look at real topography and landmarks to plan the action and movements of your people.
You can lift character names from the map and region. Swansea; Wenden; Havasu; Nothing. For my character - dibs on Chubbuck.
My guess is that you live in Fresno. You can go on a research road trip and check out the scenery in person. A great excuse for a road trip!