This is a history of the Bureau of Reclamation and the Colorado river since the early 20th century. Allocation of Colorado river water has been a contentious issue since the mid 19th century - an issue that will become more critical as global warming leads to more drought and less snowpack, and as southwest cities continue to grow, and to use water wastefully.
Here are the basic facts: despite early and accurate assessments that the Colorado would be unable to irrigate more than a very small fraction of its basin, western landowners, developers, and the Bureau of Reclamation made false claims to Congress (a Congress that, then as now, was short-sighted and venal) in order to justify the construction of dams. The initial legislation may have been well intentioned: the water diverted for irrigation was to be used only on farms of 160 acres, with absentee land ownership forbidden. The idea was to encourage migration, and to ensure that the greatest possible number of people would benefit. Of course, the land ownership restrictions were never enforced, and the result is what we have today: agribusiness megafarms, in the desert, using some 80% of the water extracted from the river. The remaining 20% of the extracted water is used by the large cities of the southwest (Phoenix, Las Vegas, Los Angeles, and Tucson), where about 50% is used by golf courses and other landscaping.
This is clearly an unsustainable situation, and was recognized as such in the late 19th century by John Wesley Powell, as he reported to Congress. And with global warming causing ever longer and more severe droughts, there is a good likelihood that even the secondary function of the Colorado river dams, electrical power, will cease to be effective as reservoir levels drop to the ‘dead pool’ level, unable to spin the electrical turbines.
Add to that the salinization of the irrigated land in the Imperial Valley and elsewhere, an effect that has brought down civilizations as diverse as the Sumerians and the Anasazi, and we can see that the southwest, and the nation as a whole, is facing a very difficult situation.
The crisis could be entirely avoided by taking most or all of the irrigated land out of cultivation; a solution that will eventually be forced by circumstances, but which would have more salutory effects if done in a deliberate way. Naturally this won’t happen, since it would require legislation and enforcement - neither of which is likely to happen in a government so beholden to industrial farming.