Cinzia,

   Why don't you try a comparison between the Mississippi for Huck Finn and
the
   Piave river for Nick Adams ?  In relation to both the characters, the
river
   is a symbolic frontier between adolescence and maturity and, if you
consider
   the historical periods in wich the two have been created, also a frontier
of
   the american experience (the american dream loosing its innocence before
on
   the western frontier, then in Europe). You can find informations about it
in:

   Philip Young
   "Ernest Hemingway"
   Ed. Mursia, Milano, 1962.

   A copy of this book is at the Braidense National Library in Milan.

   Starting from this point, you can also try a comparison between Huck and
Nick
   (the river, a geographic frontier for America) and Holden Caufield (the
50's,
   a cultural frontier and a new loss of innocence with Korean war,
Maccarthism
   and, ten years later, the Vietnam).

   Hoping that this can give you some idea,
   Best Regards:
   Luca Gandolfi