The argument to have such a deep canyon was that the forceful Colorado river could do this because it carries a lot of rocks and sand. If that would be the case, the part which is cut by the Colorado, and the part cut in by the tributaries would be very different in depth. This would lead to hanging valleys. But everybody can notice that the tributaries flow into the Colorado at the same grade, and not via a waterfall. This brought the geologists to the conclusion that you don't need all that water and all that sand to cut so deep into the canyon. They relate it much more to the arid conditions of the land, and the fact that there is no vegetation to protect the rocks. Tributaries at grade.
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