Very detailed analysis. I’ve also studied the unique topography of western North America as well. I’m not as familiar with the Grand Coulee area specifically, and that’s why this post was very informative to me. While I agree with your well thought out thesis, are you familiar with the theory of asteroidal impacts that sped up the end of the last ice age? This was for a long time a very fringe theory, but is beginning to gain some traction in academic circles.