Why does the novel begin and end at the pond?
The above is a great answer, but I'd also like you to consider the cyclical nature of the book, in other words how it begins and ends in the same place. This goes hand in hand with one of the the book's major themes "the unatainable American dream for the itinerant workers". These men work as hard as they they possibly can, only to find themselves falling short of their goals and back in the same place they started. Therefore, Steinbeck chose to have the same setting for the place we first meet George and Lennie and for where we see Lennie meet his fate. This is appropriate because despite all they've been through, they are back at the same spot.
There are several reasons Steinbeck chose to begin and end his novel at a pond. For the novels main characters, Lennie and George, the pond alongside the Salinas River is pure and untainted, a far cry from the dusty ranch where they would find work--and trouble-- the following day. The two men share a special bond and dream of owning their own place. Since ranch hands typically work alone, this time between them is one of reflection (especially in light of Lennie's recent "mistakes') and candid discussion in their quest to seek upward social mobility.
"Guys like us, that work on ranches, are the loneliest guys in the world...They got no family. They don't belong no place...With us, it ain't like that. We got a future. We got somebody to talk to that gives a damn about us"
Because of Lennie's mental limitations, he is incapable of understanding the consequences of his (past and future) actions and George, his guardian of sorts, tells Lennie to return to the river if things get difficult.
It's only fitting that George and Lennie meet again at the river, especially when George recognizes that he must shoot Lennie to save him from lynching. For Lennie, water is safe, and the commune with nature is the one place he knows things are 'ok'. It's also the restorative place where things for both men were normal. In light of Lennie's visions, he realizes something's amiss and wants nothing more than to have his puppy back and have the routine back.
Water is also thought to be cleansing and spiritual, washing away any sins (think baptism), which would work for George's final decision to shoot Lennie, a man who is like the mice he pets, weak, at the mercy of society. For George to refocus and recharge he has to be away from society's temptations (drinking and Curley's wife) while Lennie, in his childlike state, must be protected from a cruel world. The only path to redemption lies at the pond and away from civilization.
Another interesting path to explore is the significance of the water snake/heron at both the beginning and end...there is a role reversal that might play into your pond idea.
Hope that helps.