  Most nations of the world periodically publish N x N origin-destination
tables, recording the number of people who lived in geographic subdivision i at
time t and j at t+1. We have developed and widely applied to such national
tables and other analogous (weighted, directed) socioeconomic networks, a
two-stage--double-standardization and (strong component) hierarchical
clustering--procedure. Previous applications of this methodology and related
analytical issues are discussed. Its use is illustrated in a large-scale study,
employing recorded United States internal migration flows between the 3,000+
county-level units of the nation for the periods 1965-1970 and 1995-2000.
Prominent, important features--such as ''cosmopolitan hubs'' and ``functional
regions''--are extracted from master dendrograms. The extent to which such
characteristics have varied over the intervening thirty years is evaluated.
