Looks like a rationalisation after the fact really. So essentially made-up.
There's a lot of assumptions in the Grand-Canyon-formed-by-the-flood hypothesis. I don't think it's in any way probable on its own, never mind all the other problems the flood explanation has. But anyway, examining the Grand Canyon alone; it contains several different types of rocks, for example red sandstone. In simple terms, red sandstone is red because it was exposed to air while it formed. Modern geology has a sound explanation for why there are thick layers of such rock - it formed gradually and as sediment was laid down slowly there was plenty of time for the whole layer to be coloured red by oxidisation. Flood geology (and I use such a term very loosely) has no explanation.
Hypothesis: the flood formed these rocks.
Prediction: if a single event formed these rocks, they should only be reddened on the 'top' of the layer, and not all the way through, as only the top would have been exposed to air.
Testing: the evidence shows this is not the case.
Conclusion: the hypothesis is faulty and should be altered or discarded.
This isn't exactly rocket science, it's basic chemistry. Iron oxidises and rusts when exposed to air. There is iron in certain sediments. When the sediments are deposited and left exposed for a while, the iron in them oxidises and turns them red. This has apparently happened to a layer of rock in the Grand Canyon several feet thick, meaning that every time a small amount of sediment was added to it, it was left for a time and allowed to oxidise. This is not consistent with the flood hypothesis, ergo the hypothesis should be thrown out or altered to account for this evidence - unless the creationists would like to suggest that the laws of physics were different back then, which brings in the 'goddidit' factor and really makes the whole thing a little pointless.
So this is just one of many problems with the idea that the rock of the Grand Canyon was first formed all at once by the flood water, then the Canyon was carved out of them by the same water flowing away.
...Alright, being honest, the whole idea is patently ridiculous to me as a person trained in geology. And the geology of the Grand Canyon is quite simple in comparison to other areas such as, say, the Moine Thrust in Scotland.
This is a geological map of Ireland, taken from the Geological Survey of Ireland website:
Every colour there is a different type of rock. Admittedly Ireland is probably a good bit bigger than the Grand Canyon, but the point still stands I think. Try explaining this one, creationists.