Massive desalination of Pacific water should be the priority for the southwest. They could raise the money to pay for it with a tax on all Starbucks drinks above basic coffee. Might take a week or two, but the money would be raised.
The plans for this second phase canal were actually drawn up in the 1980s but the enviros killed it. The Alaska pipeline and the CAP were amazing projects from an era when America had vision and dared to think big
New desalination tech makes this plan economically reasonable to solve both California and Arizona's water needs which would take the pressure off Las Vegas
Such a plan could also allow the US to send more water downstream to Mexico