Could a massive flood have formed the Grand Canyon?
tl;dr: no.

Long answer:

First of all, like mentioned by others in the comments, you would need some physical mechanism to take a whole lot of water, evaporate it, and drop it at once at a place where the Grand Canyon is now. This is not something that's going to happen because of physics. If there was some extraordinary event which could have caused this, we would get so much more evidence from it other than the Grand Canyon.

But let's assume, that somehow, by some magic or act-of-higher-being, an enormously large amount of water fell on the Colorado plateau at once. How would that look?


The huge amount rock removed from the plateau to form the canyon would have to go somewhere. And it would go there very violently, as huge boulders, irregular conglomerates and just a whole mess of mixed up geology. However, when you look downstream along the Colorado river, you don't see this stuff anywhere. When you look in the Gulf of California (which we know very well because of oil exploration), you don't see this stuff in there. Instead you see very nice sandstones, shales, marls, clays, etc. Definitely stuff that should not be there if there was a huge flood. Furthermore, those sedimentary rocks are interbedded with volcanic rocks and limestones. If this was a sudden event, all of this could not be there.
The Colorado river is meandering. If it was a huge sudden flood you would not get this morphology.
So far I was talking about lack of evidence. There is also some direct evidence for the incision rate of the Grand Canyon. Using a plethora of of scientific methods, such as radioactive isotope dating, cosmogenic dating, thermoluminescence, etc., it's possible to directly get a time when a certain thing happened (depending on the method). This allows us to know exactly when a certain rock formed, exposed, eroded (again, depending on the method). Using these scientific methods allowed us to infer that the Grand Canyon formed in the last several few million years, at an incision rate of a few dozen to a few hundred metres per one million year. The value depends on the exact location in the Grand Canyon and also on which research group you're asking. But whether the argument is about if it's 100 or 200 metres per million years is irrelevant for this discussion - it invalidates any suggestion that it formed in one sudden magical act of flood.