As old rock, not much will happen, even the Grand Canyon remaining unaffected in spite of temporary deluges in the area. During the hour of the shift and hours following, the Colorado River will drain the immense amount of water that will dump on the Rockies, during the clash of water soaked air that will push inland into the Mississippi Valley during sloshing of the Gulf, and similar water soaked air that will push inland into the Sierras during the compression of the Pacific. Existing drainage routes will serve as the route for this water to rush back to the sea. The Hoover Dam will not survive the Richter 9 quakes, in particular will not survive the heaving and jolting that will go on as the Pacific Plates subducts under the West Coast. Such subduction does not affect only the immediate area, but causes adjustments in neighboring rock strata, sometimes hours afterwards in aftershocks. We have stated that all river will flood beyond their known flood levels, and that river banks should be avoided, and the Colorado River is no exception. Arizona will be a crowded area going into the shift due to the mystique that many ascribe to this place, particularly Sedona, and thus the poor land and minimal water will be taxed during the Aftertime. Where Arizona rises into the mountains, the highlands offer a better potential for life in the Aftertime.
Note N America Deserts commentary.
Note Arizona Lava Flows commentary.