The ending--a clue.
I could live with that ending as it is--the real climax/catharsis is Emily trusting The Indian more than anyone else.
But the credits include one for a language adviser, for Nez Perce. The Indian is a Nez Perce. Meek conjures up all sorts of danger from the surrounding red demons--he doesn't recognize his tribe or the language; he's not much of a Mountain Man either. But the Nez Perce attitude towards the strangers was mostly benign curiosity. (Which they learned later to regret, but that's another story.)
So it's a good bet that he's trying to lead them to water. The other posters who know the locale seem to think the same thing--I think him being Nez Perce cinches it.