It’s typically explained that Wallula Gap existed for at least 10 million years.
“Early in the history of folding, the ridgecrest here was slightly lower than elsewhere along the ridge. This caused first the ancient Salmon-Clearwater River (precursor to the Snake River), and then the Columbia River to flow across the ridge over this low point. As the ridge continued to bend upward, river erosion kept pace, and a water gap developed. Until about 10 million years ago only the ancestral Salmon-Clearwater River flowed through Wallula Gap. It wasn’t until about 6 million years ago that the Columbia River joined in, where it has continued to flow ever since. Somewhere between 2 and 3 million years ago, the ancestral Salmon-Clearwater River captured the Snake River in the vicinity of Hell’s Canyon, along the Idaho-Oregon border; this added significantly to the amount of water draining through the gap.”
But, if the gap existed for millions of years, how did the stratas form on the cliffs of the gap?
If my theory is correct, then it can be confirmed by verifying that the stratas on each side of the gap match the other side. And if this is so, this would be proof that Wallula Gap was created during the flood. If the gap existed for millions of years, there would be no way that each side of the gap would have identical stratas. But, it would mean that all the stratas were laid down, then the gap was created after all the layers were formed.