When do these breaks occur?

According to Cooley, physical interruptions in transportation are the most important of these causes. For example, the junction of land transportation with water transportation, or of one kind of water transportation with another, or of one kind of land transportation with another.The location of the greater number of commercial towns the world over, and from the earliest times to present day, has been fixed at the point of juncture between land and water movements. Before the railroads came in, the chief seat of inland towns has been along navigable rivers at the points where these are most accessible from the land.

Break exists wherever the technical apparatus of vehicle and forces needs to be changed. In water transportation it is found at that point in the courses of rivers where sea–going vessels must be exchanged for lighter and shallower craft. This seems to be the chief factor in determining the location of many commercial towns. They are situated upon the estuaries of navigable rivers, or upon the rivers themselves at a place not too far up to be accessible to large ships. The break is between two varieties of water transportation rather than between land and water, an exchange between boats built to slide over shallows and those built to cut the waves. Many great rivers have produced cities of this sort such as Alexandria and Cairo on the Nile, the Babylon  on Euphrates, Hamburg on the Elbe, London on the Thames, New Orleans on the Mississippi, New York on the Hudson.  Cooley points out that cities famous for their commerce but not situated on the estuary or lower course of a navigable river are comparatively few. He also noted that the lake ports are among the American cities were experiencing most rapid growth. The examples are Chicago, Buffalo, Detroit, Cleveland. New York’s importance lies in its location at the juncture of both land and water terminals.

Therefore, at the mouth or key points of rivers, meeting points of hills and plains, and other such areas that commercial city formation appear.

**Note: A good material for discussion could be Jim Corbett’s eaasy title "Life at Mokameh Ghat".