When you go to turn on the tap and nothing comes out, the next thing that happens is that economic activity grinds to a halt, much as it does when the power supply goes down. Who wants to visit Nevada if there's no water in your hotel to take a shower, just drinking water only?

Now, maybe if the snowpack is gone and the rivers are dry someone could set up a gigantic solar desalination plant on the California coast and start producing water to be piped to Nevada and Arizona... which would need more gigantic solar plants to produce the electricity to power the pumps. Someone will have to pay for that, and the costs will be pretty high. Probably higher than the revenue from the tourism or agriculture in the region. What happens then? People leave the region.