4. why will some counties grow and others decline in population?

A population decline (or depopulation) in humans is a reduction in a human population caused by events such as long-term demographic trends, as in sub-replacement fertility, urban decay, white flight, or rural flight, or due to violence, disease, or other catastrophes.[1] Depopulation can be largely beneficial for a region, allocating more resources and less competition for the new population, in addition to exempting the disadvantages of overpopulation, such as increased traffic, pollution, real estate prices, and environmental destruction. Per-capita wealth may increase in depopulation scenarios[2][3][4], in addition to improvement of environmental quality-of-life indicators[5] such as improved air and water quality, reforestation, return of native species, etc. The accompanying benefits of depopulation have been termed shrink and prosper,[6] with benefits being similar to the post-Civil War Gilded Age, post-World War I economic boom, and the post-World War II economic boom.

Hey this is your answer

Answer evaporation and condensation;