Mark Twain will tell you: a river is a river is a river:
The Mississippi River will always have its own way; no engineering skill can persuade it to do otherwise...
- Mark Twain in Eruption
In The Adventures of Huckleberry Finn, the Mighty Mississippi is, at first, a symbol of freedom. The river is a realistic boundary between two very different "countries" (North and South, free and slave states). It stands in sharp contrast to the land, where slaves must toil. It juxtaposes the static society that wants to keep slavery as an institution. Whereas the land has laws (even unjust ones), there's no laws on the river. It keeps moving right along...
The river eventually becomes a red herring--a false symbol of freedom. It fogs as Jim and Huck pass Cairo, their turn to the Ohio River. It carries them one way only, South, deeper into the dangerous slave states. So, whatever symbolic or mythological significance it might have carried is washed away.
Remember, Huck Finn is a comedy, which is the very opposite of mythology and romance. Comedy is rooted in realism. So, the river ends up being a river. We might have thought, like Tom, that is stood for freedom or religious absolution in the beginning, but in the end, it ends up being a natural force that is governed by its own laws, not the gods' or man's laws.