MacDonald states that a mirage is when the sky acts like a mirror showing water that is far away to appear closer. A mirage is a refracted image of the sky on the ground that appears as water.
It is mistaken as an error that because it is stated in the beginning that Duncan MacDonald speaks Cree and that the story will be told with the Cree Indians speaking English that none of the Cree could be bi-lingual. In particular when Duncan's adopted Indian son has a conversation with the captive white woman.
Montana in this movie is actually Sedona, Arizona.