I’m not disagreeing with any of your major theses about where and why cities grow, I’m just picking at one detail. I use the Census Bureau’s definition of the Great Plains, which starts well to the west of all but two of the Missouri River cities you listed (Pierre, btw, has finally grown to all of 14,000 people). If excluding the much wetter prairie regions to the east of the Plains is peculiar, so be it. I’ve lived in both, and find that there are significant geographic and cultural differences in the two regions. Part of the reason for the tribes wintering over in the area of what is now Denver is weather-related — the stretch of Colorado east of the foothills and between the Palmer Divide on the south and Cheyenne Ridge to the north has much milder winters than would otherwise be expected. Go a hundred miles straight east (or 20 miles west into the foothills) and the winters are much worse. As you say, unique geography matters.