Read up on the Colorado River Compact. Where the Water Goes by David Owens is a very accessible primer. The tl;dr is that the water was portioned out to the Western states (including Arizona) during an unusually wet period, and we're now in a period of drought. They simply didn't understand this in 1922. With the advent of dendrochronology, we now understand that this river system is prone to droughts that can last hundreds of years.
https://en.wikipedia.org/wiki/Southwestern_North_American_me...
