What is the background behind the photograph "The Three Chiefs" by Edward Curtis?

"The Three Chiefs" is an iconic photograph taken by Edward Curtis, an American ethnologist and photographer who dedicated his life to observing and photographing various Native American tribes and the landscape/ culture of the West. "The Three Chiefs" was taken in the summer of 1900, when Curtis accompanied George Bird Grinnell on a field trip to Montana.
Through his photography, Curtis came to believe that documenting the language, music, history, mythology, and customs of North American native peoples was of utmost urgency given the rapid erasure of their way of life by encroaching Western influences.
The photo itself depicts three tribal leaders wearing traditional clothing and posed on horses in the midst of an expansive prairie landscape. It took three days to secure the "perfect combination of riders, sky, and prairie." Curtis's website describes "The Three Chiefs" as "undoubtedly the single most important of Curtis' 50,000 photographic images." 
I have attached an image of the photograph below.