Many cities in the Megalopolis region, such as Baltimore, Washington, and Philadelphia, are located on the Fall Line, why were so many settlement established on or near this line?

Explanation:

A fall line is an area where a coastal plain and upland region meet and prominent rivers cross it that results in waterfalls.

The coastal plain is made of softer sedimentary rock while uplands are crystalline basement rock. In US fall line is a 1,400 kilometres escarpment line where the Atlantic Coastal plain and Piedmont meet

The Eastern seaboards fall line was important for the economic development of the US because the elevation change along fall line allowed the settlers to use the water as a source of energy. The mills also used machinery which was powered by water.

Pennsylvania, Philadelphia, Virginia, Richmond, New Jersey and Trenton were the cities that developed along the fault line. .  

i believe the answer is: returns on investments, no wages

people who belong in upper class society would most likely get there by obtaining multiple source of income, which a certain percentage of ownership for businesses. since most of them are job creator, they would most likely get their money from profit, not wages.