From Calif, to New Mexico, to Wyoming & west of The Rockies, lack of water is one of the best reasons, aside from invasion from the west, that military forces of US just might block interstate migration from west to east at The Rockies. If many more people move eastward, water will be depleted there as well.
Speculation: Once few people left west of Rockies, will be likely that wells, lakes, etc. begin to fill again, as Billy needs it for his crops. The Mexicans that will be allowed northward might be restricted to “the western corridor” in order to help with harvesting of Billy’s crops. There’s more to this, such as restriction at The Ozarks & Tennessee Valley, but, another time…
