The Census Bureau's weekly data visualization shows the change in population density by county of the United States each decade from 1920-1950.  In the 1920s, predominantly rural counties in the East saw declining population densities due to a trend of migration to cities.  The 1930s saw the Great Plains decline in population while rural areas in the East became more densely populated again.  As World War II ended, the United States saw increased geographic mobility, which led to 21.5 percent of civilians moving to different counties or states between 1940 and 1947.  The map shows that urban and populated Northeast and Midwest counties saw the largest growth in the 1940s, while predominantly rural counties in the Great Plains and the South continued to see declining population density.