Brad Udall at the Western Water Assessment has written an extremely helpful summary of the history of research into the effects of climate change on the Colorado River. It is interesting to note a consistent message in nearly 30 years of work on this question: the sign is nearly always negative. There’s not a whole lot of reason to think the Colorado River, which provides water for seven states in the western U.S., is going to get wetter over the coming century, but much reason to think it could get drier.
In particular, Brad includes an accessible discussion of the latest work by Niklas Christensen and Dennis Lettenmaier at the University of Washington. Their modeling shows a decrease of 6 to 11 percent in Colorado River flow over the next century on average, a change dominated by temperature increase.