Just to throw a couple of more old pics out there and its just a cool story. Thirty years after the battle, the Old West was long over, three of the survivors would meet in 1898 to see if they could find the battle site. They did and they erected a stone, later the monument would be erected and the building.
The pic below is about the oldest i have seen of Beechers it shows what the river would have looked like at the time hardly any trees, no cover just rifle pits dug behind dead mules and horses on a small sandbar in the Arikaree river.
The little sandbar in this pic is pretty much where it went down.