Yeah, that’s a good question, because though there is a Two-Hearted River in Michigan, it isn’t the river Hemingway is writing about. Here’s Hemingway in a piece he wrote called “The Art of the Short Story” (Source) to give us some answers:
The river was the Fox River, by Seney, Michigan, not the Big Two-Hearted. The change of name was made purposely, not from ignorance nor carelessness but because Big Two-Hearted River is poetry, and because there were many Indians in the story, just as the war was in the story, and none of the Indians nor the war appeared. As you see, it is very simple and easy to explain.
Alright, Hemingway is kind of being a jerk here, so don’t take him completely at face value, but he didn’t change the name just to mess with his readers. If anything, we need to pay more attention to the title now because it’s not just a place-name.
The actual Two-Hearted River is in the upper peninsula of Michigan, which is nowhere near Seney. In Hemingway’s defense, though, “Big Fox River” doesn’t have the same ring to it. So Hemingway liked the sound of Two-Hearted better, but why did he choose this one particular river to reflect his story, and why did he add the word “Big”?
We’re not going to get into why anyone would name a river Two-Hearted, because we are neither etymologists nor geographers, so our first question (we’re going to be asking a lot) is: What does it mean for something to have two hearts, symbolically speaking? It could mean that they are conflicted, as in torn between two wills. It could also mean that they are bursting with emotion. Both of these ideas seem to apply to Nick. He wants to be all stoic and in control all the time, but sometimes clearly isn’t. Remember how sensitive he can be to random little things.?
Now how about “Big”? Well, big connotes something that is possibly too, um, big to really handle. Think of the big trout that Nick fails to catch. Here’s what he says about it:
By God, he was a big one. By God, he was the biggest one I ever heard of. (II.33)
So there’s the “Big” of the story—or at least, there’s one of them. “Big” can also connote a lot of something… a lot of river, a lot of emotion, a lot of baggage. Size matters here; this is not just a little case of the blues that Nick can vacation away. Word by word, this title is talking big, doubled, emotion.