The most obvious symbol of the river is the freedom that it gives both Huck and Jim. One of the freedoms is that nothing matters while on the river; they are free to do whatever they please. They don't need to be civilized, schooled, made to wear certain outfits at certain times, or anything else that they don't want to do. They can sit naked and nobody can say a thing about it. To Huck, the river represents just that very thing: no civilization or rules. But to Jim, it represents much more: the freedom that he will soon have. The river will lead him to that freedom.
The land, on the other hand, is the opposite of all of that. When they have to go to land, a lot of worries, lies, and disguises have to form in order to stay there. They witness murder, witness and feel the injustices, and have to deal with authority. When Huck went to the Grangerford's house, he was automatically forced to be "sivilized" like he had when he was with the widow. When Jim and Huck are on land, they have to pretend to be people that they are not. Sometimes they might physically pretend to be somebody else, like Huck did with Miss Douglass, but mostly its that they have to act a certain way, a way that is not their own. The river episodes, for the most part, are all carefree, whereas the land episodes are all very tense.