Symbolism, Imagery, Allegory
So we know that Twain had a thing for rivers. Sure enough, in Pudd'nhead Wilson, the river plays a major role. Not just a feature of the landscape, the river in this novel symbolizes some of the darkest aspects of slavery.
The river starts off innocent enough. In fact, it first appears as the centerpiece of this quaint little town:
The hamlet's front was washed by the clear waters of the great river [. . .] Steamboats passed up and down every hour or so. (1.4-5)
Perfectly lovely, right?
Before long, though, the river comes to be associated with Doom (yes, with a capital D), as we find out in the scene in which Percy Driscoll threatens to sell his slaves down the river. It turns out that being sold down the river is just about the worst thing that could happen to a slave. Check out Percy's threat:
"[. . . ] I will not only sell all four of you, but I will sell you DOWN THE RIVER!"
It was the equivalent to condemning them to hell! (2.36)
Yikes.
This terrifying specter of the river haunts Roxy even after the actual threat of being sold has passed:
A profound terror had taken possession of [Roxy]. Her child could grow up and be sold down the river! The thought crazed her with horror. (3.1)
And the image is so powerful that it prompts Roxy to enact her baby-switching scheme.
But we're not through with the river just yet. Later in the novel, mean old Tom betrays Roxy's trust and ends up selling her down the river. She's totally shocked at this turn of events, as she exclaims:
"Oh de good Lord God have mercy on po' sinful me—I's sole down de river!" (16.16)
Poor Roxy, indeed.
After her harrowing experience down the river, Roxy confirms that, yes, this place is just as miserable as its reputation. She lets Tom have it:
"Sell a pusson down de river—down de river!—for de bes! I wouldn't treat a dog so!" (18.9)
Keeping these examples in mind, the river is a pretty apt symbol for the instability of a slave's situation. Think about it: rivers can be calm or rough depending on factors that are pretty much out of human control (like the weather).
In a similar way, the slaves' lives could be just as unstable and unpredictable. As pieces of property, even slaves working under relatively mild conditions could easily find themselves in rough waters, so to speak, according to the master's whims.
We don't know about you, but the river in this novel is enough to make us a little seasick.