This is an interesting issue for young people today. I'm too old, but young people will have "fun" dealing with land west of the Mississippi being in a massive overpopulation bubble, and folks east of the Mississippi having more water than we know what to do with now, and with climate change we're supposed to have even more water leading to some interesting infrastructure problems.

For a bit less than 50 years I've heard continuous and constant boosterism that the burbs will migrate back into the city center. No really, this time we're serious about it, etc. Its a cliche now. However a much more interesting migration, is over the next generation or two, most of the west half of the USA will have to migrate back to the east half, assuming they want to drink water or grow plants. Its going to be interesting to watch, at least for today's younger kids to watch.

Note that the land will support some people. In 2030 the west is not going to be empty, any more than it was empty in 1830. It just won't have as many people as now.

I doubt it. Several factors spring to mind that will prevent that specific mass migration:

1) The use of a (small) part of the west's capital to pipe water in from further away

2) Opportunities for multiple orders of magnitude better conservation.

3) Desalination

None of those are as easy as "just wait for rain" but as long as it's cheaper than the entire state of California ($10 trillion?) then it's not a financial hardship compared to "everyone just leave."