[ChuckMcM]

http://www.sacbee.com/news/local/article132356269.html provides an excellent description of the challenge. The hillside where the emergency spillway is would be at risk.

[verytrivial]

Excellent link, thanks. I found the following comment sobering:

"Countryman: It’s not going to be the (main) embankment failure, but it’s a failure. If it does happen, there’s nothing saying that the ground is going to stay where it is. That force of water will start tearing that hill apart​,​ and it could eat back into the reservoir and drain the reservoir."