How does the Mississippi River act as a "spine" in the Adventures of Huckleberry Finn?

In Mark Twain's novel, Huckleberry Finn's adventures are more like vignettes than chapters; the flow of a single story is difficult to pin down, and some readers find this confusing and frustrating. The best analogy for Twain's style with this novel is to think of it like a TV show in which each episode features the same cast and many of the same themes, but the setting or conflicts might change: Scrubs, Law and Order...

In Mark Twain's novel, Huckleberry Finn's adventures are more like vignettes than chapters; the flow of a single story is difficult to pin down, and some readers find this confusing and frustrating. The best analogy for Twain's style with this novel is to think of it like a TV show in which each episode features the same cast and many of the same themes, but the setting or conflicts might change: Scrubs, Law and Order SVU, 30 Rock, The Office, Master of None. These are all TV shows whose episodes slowly build characters and themes rather than an overarching plot, which happens in shows like Jane the Virgin, The Unbreakable Kimmy Schmidt, The Leftovers. 

However, over the course of the novel, there is one connective setting that ties each vignette (or episode, if you will) to the next and develops the main characters (Huck and Jim): The Mississippi River. Acting as a proverbial spine, each vignette branches from the River, which itself features much of the character and thematic development of the protagonists. Without the River, the story would have no rational means of connecting each story with the next - no backbone.