The river, and in particular the Mississippi River, is an appropriate setting for a story of American culture and of a struggle to attain independence of thought, spirit and physical being.
The river is a passageway, a highway of sorts, and a natural limit marking the boundary that separates the developed and civilized east from the wild and loose west. Huck travels on this border contemplating freedom and experiencing a conflict of conscience that is directly related to a struggle to accept or reject social norms associated with "civilization".
As a main thoroughfare of American commerce, the river is also a natural place to encounter various elements of the culture that characterizes the nation and with which Huck is attempting to come to terms.
The nature of the story as an adventure also is in keeping with this setting and its scope of possibilities.