My goal here is to describe a world which works as a setting for role playing; a world which is fun to play in, and easy to run. I have not tried to construct a historically accurate world, or a school text.
This setting is Mythic in much the same way Middle Earth is Mythic. I combine myths from many different cultures together into one world, just as Tolkien did. Therefore you will find elements taken from plains tribes, Californian tribes, the farmers of the Southwestern US, all put together. (A “mashup” to use the trendy phrase.) This is just one of the elements that makes this world Mythic American Indian, rather than Historic American Indian.
The Indians described here are not the low population, relatively peaceful, nomadic, nature lovers you might have learned about in school or in some new age TV program. These Indians lived in huge civilizations that exploited every natural resource available to them, although very differently than Europeans would.
This world is very strictly pre-contact. You will not find any European elements here: no horses, no wheels, no metals, etc. So you won't see discussion of Spanish explorers, Mexican rancheros, or gold diggers of any nationality. If you want that, GURPS: Aztec covers Spanish explorers, and GURPS: Wild West covers some of the rest.
I have focused on California tribes in creating this setting, because that is where I live. However, I think many of the ideas here can be applied to any American Indian group. And you will see references to non-Californian groups.
It is important, but difficult, to realize just how primitive California Indians were. (Perhaps “low tech” is a better word than “primitive”.) They are often described as "stone age", which is true, but insufficient. They did not farm (in most of California) or use the wheel. They had no beasts of burden or horses (which came across with the Spanish).