As someone who grew up near there, and has family and friends still nearby, allow me to clarify.
The Auxiliary, or Emergency, spillway is at risk of failing due to extreme erosion. If it does, fail, it will open a 30 foot high breach that will allow tens of thousands of acre-feet of water to flow out of one of the largest reservoirs in California.
This emergency spillway has NEVER been used in the entire history of the dam, and they were unsure of exactly how it would behave, as this dam FAR predates computer modelling of fluids.
That said, the dam's operators have opened the damaged primary spillway to allow 100,000 cubic feet of water to flow out every second in an effort to lower water levels enough to stop flow over the emergency spillway. As of 7:32 PM PST, this has been successful, and they have lowered the lake level by more than a foot, and are expecting flows across the emergency spillway to cease within the hour. However, this is placing a lot of pressure on the levees downstream, which have not had a stellar record when dealing with flows of this magnitude.
Please note, the dam proper is NOT IN ANY DANGER. It remains in good shape and will remain so for the foreseeable future. The pictures that Andy posted are of the main spillway, and the damage suffered to it is the entire reason for the situation getting as critical as it has.
---------- Post added at 20:19 ---------- Previous post was at 19:33 ----------
Live news and reporting feed from a news station nearby (Sacramento, CA, which is a few hours downstream from the dam...)
http://www.facebook.com/GoodDaySacramento/videos/10154634310433005/
Press conference scheduled for 9:00 PM PST. As I type this, it's currently 8:18 PM PST.
As of 8:00 PM PST, lake level stands at 901.02' above sea level, which halts flow across the Aux/Emerg spillway.