Symbolism, Imagery, Allegory
For a body of water, we sure do get to hear a lot about the river:
The river was there. It swirled against the log spires of the bridge. (I.2)
He knew where he was from the position of the river. (I.11)
At the edge of the meadow flowed the river. Nick was glad to get to the river. He walked upstream through the meadow. His trousers were soaked with the dew as he walked. After the hot day, the dew had come quickly and heavily. The river made no sound. It was too fast and smooth. (I.23)
And that’s just in Part I. The river is something that Nick can count on, like a certainty, or like a State Farm commercial. Now, while Pocahontas might say that you can’t step in the same river twice, for Nick it provides all sorts of comfort precisely because he knows that it will always be there in the same place.
See, change isn’t Nick’s thing—at least, not with the way he is in this story. Things like the burned town of Seney, or the sudden movement of the trout, seem to really bother him. That’s because change is unpredictable, and you can’t really do much about it. But the river is always there. It’s a constant that Nick can use to establish his own position, literally and metaphorically (literally because he’s using it as a guide; metaphorically because he re-asserts his sense of self by knowing where he is and where the river is). Even if he can’t rely on time to stand still or rewind, he can count on the river always being in the same place. We might also add that the river is part of the natural world, which is sometimes used as a trope for simplicity and healing when civilization gets too complicated and ugly.