The solution for Vegas, given the vast amount of people and money there, will be to build coastal desalination plants and pipe the water inland. It'll cost a lot, and they'll do it in the next 30 years. It might be done on a 50/50 basis with California, sharing the plants.

A lot is a relative term though, is $50 billion spread out over 30 years too much to ensure Las Vegas has water? No, it's very cost effective in fact. San Diego's big new plant has a billion dollar price tag, but provides enough water for 300,000 people. Las Vegas has 1.4 million people in its metro area. In the grand scheme of things, this is a comically trivial problem to solve.

I have a counter prediction: Las Vegas will finish the century as the same engineering dependent marvel it is today. Except it'll be even more impressive.