Most of America possesses urban areas that have grown in population and extent. In a few instances, the growth has been so great and the size of the core cities has become so large that major urban areas have merged and formed clusters of cities. Southern California, from San Diego to San Francisco, is offered by some observers as yet another set of urban areas that will be merged by the end of the 20th century, as is much of east coastal and central Florida. Most large urban places have developed where transportation routes connect with each other. Quite often it is the land-water connection that is important.
Some urban centers are on a seacoast or large estuary.
Others are on naturally navigable waterways. Still others are on rivers or channels that have been modified extensively just to give the cities water access. However, where goods and people must transfer from one form of transportation to another, there are opportunities to process, exchange, manufacture, repackage, sell, and buy goods. Atlanta, for example, located at the southern tip of the Appalachian Mountains, had become a key inland center for railroad transportation in the South by the 1860s. Some argue that one of the great strengths of the United States is that it is the world's largest and most populous country joined geographically and socially by a common language.