List what you consider the most significant factors underlying American economic growth in early 1800's and explain why you think so.

In my opinion, the major factor (above all others by a long way) is the expansion of the transportation network in the United States.

During the early 1800s, governments in the US started to create a larger transportation infrastructure.  Roads were built and, more importantly, canals were created.  The canals, especially, allowed products to be moved from place to place much more efficiently.  As the price of transport dropped, the size of the American market grew.

The size of markets is very important for economic growth -- this is one reason countries want to make free trade agreements today.  When transportation became available, producers could, for the first time, have access to a market that stretched far beyond their local area.  Producers in the inland US could sell to a much larger market than ever before.