When Huck and Jim spend most of their time on the river, their adventures occur mostly on land. Clearly, their lives and society's expectations are vastly different in these two locales. How so? More important, what is Twain suggesting by creating such distinctions? Describe life on land and life on water for Huck and Jim.
The river is closely associated with nature, as pointed out above. Huck remarks repeatedly about how beautiful natural things are when seen from the river. He watches lightening storms and basks in the glow of the stars, acknowledging his sense of beauty and grandeur in these moments.
The land/river contrast is reminiscent of the basic philosophy of Romanticism, that evil is found in society and that goodness is found in nature. This philosophy is explored in depth by both Emerson and Thoreau. Emerson's essay, "Nature," and Thoreau's master work, Walden, both express it. Emerson said we return to "reason and faith" in the woods [nature]. Huck escaped from the corruption of society and returned to reason and faith when he rafted down the river. On the water he found his own spiritual compass, and it was true.
There is not much to add to post #2 because Twain obviously uses the river to express freedom and a sort of inner peace for Huck and Jim. However, one additional contrast is that, for the most part, Huck discovers truth when he is on the river. Instead of have others interpret for or force their version of reality upon him--as they do on land--on the river, Huck is able to have meaningly conversations with Jim about the world in general and consider the difference between what is morally right and socially acceptable.
Life on land is dictated by society's standards and rules. Their adventures happen on land because they are not living by society's rules, and that creates conflict. The river is peaceful and can be viewed as a retreat because it is where they can be themselves. The river is a place for individuals to set their own standards, and to be free from the judging eyes of those who might disagree with those choices. Twain's focus on the differences shows his consideration of the individual over the society and his belief that each person should live life according his or her own conviction and not what others decide.