Creationists seem to have an odd fascination with the Grand Canyon - probably because it so simply demonstrates the age of the Earth. After all, how many millions of years does it take for a river to erode enough rock to form the Grand Canyon? The answer is somewhere between 5 and 15 million years.
I often hear that the vast amounts of sedimentary rock around the Grand Canyon were laid down by the flood. This is easily debunked as a single event would lay down a single layer of rock. However, everyone knows that the Grand Canyon is made out of many layers of sedimentary material. Similarly floods and rivers tend to erode rock, not lay it down.
However, if that's not enough, in a layer of the Grand Canyon known as the Redwall Limestone, there exists coral fossils. So unless the flood lasted for hundreds of years and turned the Grand Canyon into a tropical paradise, there's no way a creationist would ever expect to find coral buried in there.
By the way, the reason why we find sea fossils in the Redwall Limestone layer is because that area used to be under a tropical sea around two and half to two billion years ago.
Also, it's interesting to note that all the fossils found in the Grand Canyon are separated by layer. So you don't find sea creatures in any other layer than the Redwall Limestone - impossible if all the creatures were buried at once.