Description Plains Indian, member of any of the Native American peoples inhabiting the Great Plains of the United States and Canada. This culture area comprises a vast grassland between the Mississippi River and the Rocky Mountains and from the present-day provinces of Alberta and Saskatchewan in Canada through the present-day state of Texas in the United States. The area is drained principally by the Missouri and Mississippi rivers; the valleys of this watershed are the most reliable sites from which to obtain fresh water, wood, and most plant foods.