Here is the problem with a massive flow of water cutting the canyon. If the sediments are soft they will not make vertical walls of any appreciable height:
Mass-Wasting
In fact wet sediments form very low angles of repose:
"When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid. This is because the water gets between the grains and eliminates grain to grain frictional contact."
So there goes the claim of wet unconsolidated sediments. You can't have fast erosion that way.
Worse yet, the incised meanders in hard rock show that it took a very very long time to form the canyon:
AS River Rejuvenation
The combination of both elements in the Grand Canyon shows that the
cutting of the Grand Canyon was not a slow process at all.