The author uses the river as a symbol to represent the opposite of what it usually signifies. Traditionally, a river, or water, would stand for life, cleansing and renewal, but in Rumble Fish, the water is squalid and filthy, representative of death and ruin. Early in the story, Rusty James throws his cigarette butt nonchalantly into the river, remarking that the river is so spoiled already, it will not make a difference. He later comments on the river's bad smell. The river parallels the lives of the boys who run loose in the community. In its ruin, it has only death to look forward to; it is sluggish and stagnant, and does not, like it might be expected to, flow freely into anyplace that holds more promise.
The fact that the Motorcycle Boy wants to free the fighting fish by releasing them into the river indicates that he still harbors a spark of idealism and of hope. Somewhere, he has held onto the idea of the river as a place of freedom and cleansing, and he envisions that the fish will have a better life if they are set loose in the water. Sadly, the river is so polluted that the fish will most likely die quickly in its toxic waters; like the small town where the boys live, the river does not offer the possiblity of a better life, it only leads to death. It is ironic that both the fish and Motorcycle Boy die at the edge of the river. It is as if they were striving to reach a place where they might find salvation, only to come up a little short; but the tragedy of the whole situation is that, unbeknownst to Motorcycle Boy, the river is dead, and there is no salvation there.