Something I came across on the Census Bureau site:
Under normal circumstances we would expect that natural birth rates would replenish the existing population, wherever they may be. For example, let’s take a small county with 10,000 residents. Let’s say there are exactly 5,000 women and 5,000 men and all are married and have 2 children. After those original 10,000 residents are dead, there will be 10,000 residents still, given there are no people moving into the county or out of the county. This is called the replacement rate of growth. What the above statistic is saying, is that more than half of all U.S counties are losing population.
A deeper dive would tell us whether this is an anomaly historically or not, but this seems crazy regardless. With the baby boomers reaching the end of their lives in rural or less desirable places, younger generations are going elsewhere. What the statistic really says is that the U.S. is geographically concentrating in a pretty significant way. Economically depressed counties will continue to lose population to larger, more concentrated counties, which compounds the problems these counties face. It is a self-exacerbating problem.