What are the literary elements that contribute to this story as a whole?

Literary elements are like scientific elements in that the two are the basic building blocks of stories and the universe respectively. "Big Two-Hearted River" by Ernest Hemingway is an example of the Iceberg Theory, a style that offers a sliver of information while implying deeper significance. The story follows the protagonist, Nick Adams, as he hikes, camps, and goes fishing. At face value, it seems as basic as it can get, yet there is much more.
Literary elements such as character, plot, setting, and symbolism are purposefully constructed to get at the heart of the piece.
There is only one character, Nick Adams. He is alone with himself. The focus rests solely on his actions, and how his trauma impacts the simplest activity like fishing.
The plot is sparse with little drama. This minimalism places emphasis on what tension there is, such as Nick's reaction to breaking his fishing leader, as well as the dread he feels for the disorder of the swamp.
The setting is sometime after World War I. We open with a brief glimpse of a train disappearing in the distance. Nick Adams is left by himself to take in the surroundings. The story notes how the town of Seney had been utterly destroyed by fire. The only remains are some damaged stone. While it is not explicitly mentioned, a clear parallel can be drawn between the destruction of Seney, and the bombed-out cities Nick witnessed in Europe.
Because meaning is not outright stated, symbolism is a vital literary element for this short story. The fishing site is called "Big Two-Hearted River" because of the two reasons Nick has for coming. He does not just want Trout. Nick wants to fish, but he also wants to recover from the horrors of war. In his camp, Nick is in control. He can make delicious food and smite mosquitos. This stands in contrast to the swamp, which lacked sunlight and order.

In the swamp fishing was a tragic adventure. Nick did not want it. He didn't want to go up the stream any further today.

The swamp is a symbol for Nick's shell-shock or PTSD. He is uneasy with the swamp's presence but must learn to live with it for the rest of his life.