That's a novel idea, but the Flood, while hypothetically powerful, would not consistently flow for long enough to form such a deep crevasse. 40 days and 40 nights wouldn't be enough time for any river to erode rock that much, not to mention that after a certain point in the flood story, what would eventually become the Colorado River would completely engulf the entire area, stopping the eroding flow. I mean, the Flood was supposed to cover mountains; I don't think Nevada was above water.
Yes, the Grand Canyon was caused by erosion, but it was the erosion that you get if a river (the Colorado River, to be exact) runs across the same land for millions of years, slowly wearing it away bit by bit.