This old classic, Powell's design of the Western arid states done around river basins/drainage basins, to minimize water conflict in the new states. Doubtless would need adjustments to be practical, but the general philosophy is useful, and we may be sorely wishing that Powell got his way in a few decades.
Does anyone have a WorldA of this layingaround? Or if not, would anyone be so kind as to make one?
The best possible division for the Great Basin is to just see where the endorheic basins would overflow if filled. There's really basically 2 - Colorado, and Columbia rivers as potential end-points.