From an interesting Sammy Roth interview with former Interior Secretary Ken Salazar:
Q: In Southern California, water managers often say they don’t have to worry about losing access to Colorado River water, because of the state’s “senior” water rights. During an extreme shortage, they believe, Arizona and Nevada would lose much of their river water before California loses a drop. Do you think that’s how an extreme shortage would actually play out?
A: I think California has to worry a lot about, because it has a major dependence on the Colorado River. It has to be one of the cardinal concerns of the state of California.
Salazar, who as Interior Secretary used to serve as the “watermaster” on the Lower Colorado and therefore presumably has some understanding of the legal nuances, doesn’t say how California might come to no longer enjoy those senior rights.
We’ll leave that as an exercise for the reader.