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. The climate is continental, with annual temperatures ranging from below 0 °F