A new study in the journal Science shows "that up to 60% of the climate-related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States."
The authors observed that "between 1950 and 1999 there was a shift in the character of mountain precipitation, with more winter precipitation falling as rain instead of snow, earlier snow melt, and associated changes in river flow" in which the river flow is relatively increased in the spring and decreased in the summer.
While only a small part of Arizona is among the three basins studied, the Colorado River basin is the source of much of the runoff supplying the Colorado River.