Basic reality, you look at any sort of a real river which is more than a mile wide, the Volga, Amazon... and you expect it to be shallow, at least at the edges. I mean, you don't expect to take three steps into the Volga and then go straight down 1500'. Nonetheless, that's precisely what you see on the edges of the Grand Canyon, is this unbelievable vertical drop of 1500 or 2000 feet, sharp pristine edges everywhere you look mesas which would have been 2000' below the surface with sharp, pristine edges and features everywhere... Rivers just don't do that sort of thing. Moreover the topology of the river is basically fractal, with sinuous rills just everywhere and thousands of micro-tributaries if you want to call them that; you don't see that sort of thing with real rivers.
Basic bottom line, the Grand Canyon was blasted straight out of the rock by a fantastic electrical discharge between this planet and some other cosmic body, a comet, asteroid, or another planet, and most of the material from the canyon was either vaporized or blasted straight out into space. Real interesting spectacle, but you'd have wanted to watch from a considerable distance...