My guess is underslab erosion. We can see now that the missing section of spillway was built on what I’d call lessor rock. All other issues appear to be secondary to that. In my opinion, of course.
Looks like they restarted the plant and are running it along with the spillway. That’s good to see.
http://www.water.ca.gov/news/newsreleases/2017/031817_dwr_news_elease.pdf