The scour shown in 617 shows though there is rock, it is badly fractured and jointed allowing water to dislodge and move it away. The photo that is most telling for me is the one in #589 which shows how close the scour came to the north-end base of the emergency spillway - maybe only 25-30 feet based on the workers (shown in the picture for scale).
No planned time to lift evacuations near Lake Oroville
No estimate yet for evacuees.
Here’s some more pics of the scour area.
http://www.sacbee.com/news/state/california/water-and-drought/article132463009.html