http://www.republicoflakotah.com/wp-content/uploads/2008/12/sioux_reservation_map.jpg
This will help you to better understand the novel. This shows the land of the Great Sioux Reservation since 1868. It also tells you the setting of High Elk's Treasure, since Joe lives on a Sioux reservation. It also shows the land that was lost from 1868 to present day. You can also see some rivers on the map. The creek that Joe loses Star on might lead to one of those rivers. If you have a map of the area, you can visualize the scenes in the book better, and seeing the "movie in your head" might help you understand the novel.
My reaction to this map was sadness. It doesn't seem fair that the original inhabitants of America get a small piece, while the Europeans and settlers from other countries, come in and surround and take the land. I would hope that people would respect and understand the importance of Native Americans, and give them land that originally belonged to them. But you can see that the land area grows smaller and smaller for the Great Sioux Reservation.