This is a satellite image of the reason I was headed into Washington this time. What you are looking at is evidence of repeated catastrophic flooding events. This is how the theory goes: During the ice ages of 13k - 15k years ago, massive glacial lakes formed during the melts and when they let loose a sudden barrage of water. They estimate that the largest of these was the equivalent to 13 Amazon rivers and they weren't selling any books.
So use your imagination a little. The floods were coming in from the top of the image. Perhaps at first the flood fanned out over the plain tearing up as much as it could. Then in the area over to the right a fissure in one of the basaltic layers gave out... maybe there was a cavern underneath carved out by millennia of seepage, and the weight of the water caused it to fail creating a deep channel that pulled away most of the water... well I took a few more pictures of what I saw...