After the Great Nuclear War, North America experienced several large migrations of people from regions no longer viable without modern technology.
Great Plains Migration
The Great Plains quickly became nutrient poor, dry, and infertile without modern technology. 5.7 million people are displaced from the region, and begin moving to other regions en masse. These migrant are referred to as "Braskies" (somewhat derogatory), and deposited into a handful of specific regions, namely, the Mississippi Basin, Colorado, and some Belt Nations. In the Belt, they are mostly considered freeloaders, but are welcomed in other places as often times they have farming expertise and other important practical skills. In addition, many Braskies have joined the major wandering cultures that have developed on what parts of the Plains are still usable, such as the Coways, and the Sioux.
Great Basin Migration
The Great Basin was largely uninhabitable even with modern technology, so the movement of people from this region is even more understandable. Sandys, as hey are known, migrated to a great many regions. Some of the most migrated to are Colorado and the Colorado River Basin in Arizona, in addition to NoCal. Some other regions traveled to are Southern Idaho, Kaibab National Forest, Northern Utah, and SoCal. Some Sandys remain in the desert as nomads, travelling from settlement to settlement, scavenging, trading, and hunting. One popular destination for Sandys is Hoovertown, one of the fastest growing western settlements, with fresh water, electricity, trade, and somewhat reliable security from nomads.