Symbolism, Imagery, Allegory
The Mississippi River is pretty honkin' big—in fact, it's the longest river system in North America and the fourth-longest in the world. It spans 2,320 miles and runs through ten states before reaching into the Gulf of Mexico. At its widest point, the river measures seven miles. In other words, they don't call it the Mighty Mississippi for nothing.
Why the geography lesson, you ask? In Gone Girl, the Mississippi is a serious metaphor for Amy and Nick's troubled relationship. Like the river itself, the conflict between them is overwhelmingly large, deep, and far reaching.
Nick himself points to the river as a symbol when he tells us that North Carthage isn't "built on some safe bluff overlooking the Mississippi—we are on the Mississippi" (1.36). The Dunnes aren't in a safe place anymore, and the troubled waters of their relationship are rising. Amy also senses that while she's on the run, things aren't what they should be: "Everywhere I go is the river. I'm following it, or it's following me" (42.1). She can run from the mess she's made at home, but try though she might she can't hide.
Even Amy's father seems to recognize the darkness the river reflects in their situation. He says:
"When Amy talked about moving back here, back along the Ole Mississippi River, with you, I pictured… green farmland, apple trees, and those great old barns. I have to tell you, it's really quite ugly here." (17.65)
It's an ugly scene for an ugly chapter in Nick and Amy's life together—and it just gets uglier. Amy is "sending [Nick] up the river" (31.25), both literally and figuratively. On the literal level, she's trying to send him to prison. On a metaphorical level, she does send him to a prison of sorts—the prison of their marriage, of being forced to stay with her terrible self once she returns.
The river is a constant backdrop in this story, so in case you haven't had enough of the Mississippi yet, be sure to check out the "Setting" section.