Buzsaw
Logically it seems the catastropic Global flood suddenly cut the Grand Canyon somewhat like the Mt St Helens canyon and there wasn't a similar sudden break through condition to cut the other rivers so they didn't get cut deep.
So why does soil like the rock of the Grand Canyon dig so deep while a river such as ,say, the Ganges does not since the same flood hit each.
How about the Nile or the Mississippi as well whose soil is soft in comparison and easier to erode?