Roxy jests that she would sell Jasper "down de river" in the same manner that Driscoll threatens to sell his slaves down the river if they do not confess to stealing.  This reference refers to the practice of selling slaves further south, where conditions of slavery were perceived as harsher than those in northern slave states.  The chapter notes in the Penguin Classics text of Pudd'nhead Wilson suggests that "slavery down river was probably worse, on large, over-seered plantations...the terror refers to the ugly inter-state slave trade, which involved a large south-western migration of slaves to satisfy the rise of cotton."