river also allows Huck to get away from his father. Huck has good reason, as introduced early on in the story:

Pap hadn't been seen for more than a year, and that was comfortable for me; I didn't want to see him no more. He used to always whale me when he was sober and could get his hands on me; though I used to take to the woods when he was around.

When Pap returns, he figures Huck has money (Tom and Huck had found gold at the end of Twain's novel about Tom Sawyer), and he wants it. Pap kidnaps Huck. Huck grows to like living off the land, smoking and "cussing," but when his father gets drunk, he beats Huck, and the boy gets tired of it...

By and by Pap got too handy with his hick'ry.

So Huck kills a pig to stage his own death. In order for the plan to work, he must leave town, and the river carries him away.

While hiding out on the island overnight, Huck discovers Jim, Miss Watson's runaway slave. Fearful that Miss Watson is going to sell him to someone in New Orleans, Jim has run away. He, too, looks to the river for a means of escape and freedom. He wants to find his family (who he has been separated from), and move North where they can be free. (New Orleans would only take him farther away from his wife and children.)

The river provides a means of escape when Huck gets mixed up with the Grangerfords and Shepherdsons. In fact, most often when they confront an emergency of some kind, the Mississippi carries them to safety.

Even at the end of the story, Huck has the opportunity to be "sivilized" again, but decides instead that he'd rather be free, and takes to the river.

But I reckon I got to light out for the territory ahead of the rest, because Aunt Sally she's going to adopt me and sivilize me, and I can't stand it. I been there before.

In the richly-textured Huckleberry Finn, characters, actions, objects, and settings often play dual roles, both as facets of the plot and as symbols of ideas. The Mississippi itself comprises the central symbol: both Huck and Jim flee to it, journeying down it in search of freedom, Huck from his abusive and alcoholic father, Jim from slavery. Their often-interrupted journey symbolizes their metaphorical “journey” through life; the dangers posed by/carried by the river itself symbolize the dangers and difficulties of living in the 19th century South. Symbolizing the fluid aspect of life, the river never breaks permanently free from the inertia symbolized by the constraints of the banks between which it flows on its way to the freedom of the Gulf of Mexico.

All travelers need food – both Huck and Jim had been undernourished, deliberately, by their “masters.” Because Jim dares not leave the raft, Huck must secure sustenance, and the act of procuring food for Jim as well as for himself symbolizes the nurturing traits in Huck’s personality, as the deliberate deprivation of food had symbolized their captors’ indifference and casual cruelty. [As Jim shows his personal care and concern for Huck, he “feeds” the boy’s unmet need for love, protection, and approbation.]

Numerous characters serve as symbols: both the “Dauphin” and the Widow Douglas symbolize the flaws in religion, the Dauphin as a manipulator who lies about religion and the Widow who sees it as her “Christian duty” to “civilize” Huck by polishing his manners rather than by genuinely nurturing him. Other symbols of the flaws in religion are the Grangerford and Shepherdson families, who regularly attend churches which preach “brotherly love” but who have been engaged in a deadly feud for over 30 years. In addition to symbolizing religion’s flaws, the Dauphin and the Duke symbolize the falsity of appearances by masquerading as royalty. Tom Sawyer, despite his genuine efforts to help Huck and Jim, symbolizes those enslaved to an unrealistic and romantic view of life; Tom’s “help” nearly gets people killed when, in order to recreate the melodrama of the romantic tales which he adores, he concocts an unnecessarily elaborate scheme to free Jim.

Many facets of the novel further symbolize the theme of appearance versus reality. Huck, not realizing that his father is dead, continues to flee from a threat which has vanished.  As mentioned earlier, the grafters the Dauphin and the Duke fool and bilk those who cannot see beyond appearances. Tom conceals the reality that Jim is now free solely in order that he can carry out his dramatic and completely unnecessary “rescue.” The Grangerfords and Shepherdsons, who profess Christianity and attend church regularly, have nevertheless been killing one another off at every opportunity for 30 years, their actions belying their Christian veneer. Huck as a girl offers a comic version of the theme, with even the sharp-eyed woman who discerns his gender despite his appearance unable to penetrate to the reality that he is not the escaping apprentice for whom she mistakes him.

One of Twain’s major strengths as a writer was his ability to incorporate and manipulate symbols while seldom descending into pure allegory; he was first, last, and always a master story-weaver.

The raft can be seen as a symbol of good fortune, having come to Jim and Huck on the island. They do not have to steal or cheat their way into ownership of the raft. It simply comes to them. 

One night we catched a little section of a lumber raft—nice pine planks. It was twelve foot wide and about fifteen or sixteen foot long, and the top stood above water six or seven inches—a solid, level floor.

The raft also serves to symbolize freedom, as this is the primary vehicle for their travel down-river. More specifically, the raft can be seen to stand for collective freedom and mobility. 

On the raft, Huck and Jim are able to stay together, living as more-or-less as equals. Often, Huck takes a canoe to go off on his own to perform a task that would be dangerous with the two of them together. The canoe can be argued to represent individual freedom, in this way (or individual responsibility), while the raft represents a more collective freedom. 

Gold becomes associated with death by the end of the novel. Huck and Jim acquire gold when they steal the canoe from the criminals on the sinking steam boat. Those criminals die soon afterward. 

Also, during the Wilks family episode, Huck hides a bag of gold in a coffin, which is then buried with the dead man. 

Though gold may not exactly represent death, it is closely associated with death in these episodes.