Throughout most of the novel, the Mystic River is an obscure entity only briefly mentioned. It is first alluded to by Jimmy: “[H]e tried to ignore images from that night by the Mystic River—the guy on his knees, saliva dripping down his chin, the screech of his begging” (96). This anchors the significance of the river in Jimmy’s past, suggesting that the connection he has to it is sinister. As the story unfolds, it becomes increasingly clearer that the river works to symbolize Jimmy’s violent past—and perhaps even his future. When he dreams of it, Jimmy has the feeling of “Just Ray Harris and the Mystic River knocking at his door” (298). The novel positions the Mystic River as cosmically linking Jimmy, Katie, and Just Ray through fate. This proves to be exactly right when the Mystic River is revealed as a metaphor for karma; Jimmy’s murder of Just Ray Harris in the river is poetically avenged when Just Ray’s son murders Katie.   

However, if the river is a metaphor of karma, then Jimmy’s murder of Dave must have its own cosmic consequences. Though the novel ends without justice being achieved, the possibility of the scales being rebalanced against Jimmy are alluded to: Jimmy claims to have had his sins washed clean by Dave’s murder, but after the deed, he “plunged his hands into the river, oily and polluted as it was” (368).