Imagine the water resulting from all those melting glaciers at the ending of the multiple ice ages.
At a minimum, 100x the current maximum flow, not to mention vastly larger floods from the bursting of ice dams.
Not that I believe in the ice ages, but if there were "all those melting glaciers" and no Grand Canyon, you'd sort of have to explain why the melting water or the ice found its way up to 5000 feet above sea level, and if it was there why it didn't flow downhill like water is supposed to do but instead cut a massive chasm (or really chasms, but that's another problem) in the rock.
ML/NJ