I found this image to be interesting because not only does it document a couple of native peoples, it also shows them in the context of their homeland, their American landscape. The two Apache scouts in the foreground appear to be at one with the environment, in relaxed poses, and although they are carrying weapons, there is no sense of threat coming from them. The calm of the lake helps to emphasise this feeling.
Led by first Lieutenant George Wheeler, the Wheeler Survey was compromised of several expeditions as the survey was carried out in order to determine what America was like across the continent. Pushing further west, their main goal was to make topographic maps of the South Western United States.
Although it may appear that the photograph has been taken to document life, the natural world, and the terrain of the unsettled parts of America, it can also be argued that the reason for the Wheeler Survey was to establish understanding of the people who lived there, how many of them there were, and also how densely populated the area was. Therefore, I think it's safe to assume that although O'Sullivan may have been taken on the expedition because of his photographical skills, and his eagerness to travel with a mobile darkroom, it is more likely than not that his purpose was to bring back evidence. The Wheeler Survey, and the expeditions that formed it, was concerned with Manifest Destiny and how the settlers could achieve it.
It could be argued that because of their mission to gather as much information as possible, these photographs were taken in their natural environment. O'Sullivan appears to have captured the scene as he found it in order to convey a realistic representation of what was out there on the other side of the continent. This is interesting because previously, reports of America have always depicted the land as being kind, and that the settlements made would be simple. O'Sullivan, however, has shown the landscape as it is. The people in the frame are vastly overshadowed by the wilderness, but the native Americans are comfortable within it.
The Apache scouts were not under imminent threat when this picture was taken, but we, with the benefit of hindsight, know that the sense of calm and beauty portrayed in the photograph did not last for long.