to clarify: it's not the dam itself that's at risk of failure, but the emergency spillway. while this scenario would be catastrophic, it's far less catastrophic than the entire dam failing
If it fails, it will be a double disaster, many people will lose property, and California will lose some or all of the water in its largest reservoir. The challenge is going to be keeping ahead of it through to the next storm.
"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."
No, it's not. The local geology is fairly solid rock. The overflowing water will wear down the overlying soil and loose Earth, but should be sustained by the rock formation. It is a risk, certainly, but such a site is chosen because of the suitable foundation.
The issue right now is to see how the spillway fares over the night. If it handles the increased flow, then it should be able to continue to relieve the situation.
To be fair, I think it is a little ethically troubling to refer to the emergency spillway as anything other than that (e.g. auxiliary spillway), but it is designed to function in an emergency, not just fail catastrophically.
Where did you find the info on the area being solid rock?
That would go a long way to giving an idea as to how bad any increased failure might be.
Lake Oroville is actually the second largest reservoir and one of six reservoirs with a storage capacity over 1M acre feet of water: http://cdec.water.ca.gov/cgi-progs/reservoirs/RES
The largest is Lake Shasta: http://cdec.water.ca.gov/cgi-progs/stationInfo?station_id=SH... with a storage capacity of 4.3M acre feet of water, approximately 800,000 more acre feet than Lake Oroville.