Your second picture only reinforces the evidence of a sharp "V seam" of weathered rock being removed revealing the hard amphibolite base. This would be normal for historic waterflows at these locations causing the weathering & thus becoming the "highly fractured weathered rock".
Keep in mind, that all of the observed eroded weathered rock is gone. Where did it go? It tumbled down the flow channel.
But don't take my word on all of this... look at the DWR geology video of the new mini-fault "seam" discovered at the bottom of the emergency spillway flow. The geologist explains how this seam was revealed by the erosion & the amphibolite that remained.
Yes it did. Water cut through the weathered brown rock and washed it away. Water continued cutting into the underling gray rock. Was some of the gray rock eroded by "weathered rock debris filled flow"? Of course, yet being softer how much damage did it cause? All of it? I don't think so. I don't dispute that the weathered rock contributed to erosion of the gray rock. I think much, if not most, of the erosion of the gray rock was caused by water and occurred after the overburden was washed away.
