It's also the best example of the use of a river in American fiction. I can't imagine how could you have taught that course without Twain! The great thing about how Twain uses the Mississippi is that it works on both a literal level (as the setting for much of the story) and a symbolic one. It's perfectly exemplifies what writing teachers like me are always telling students: Symbols must arise organically from the story. You can't superimpose them on it. Thanks so much for raising that point.