The erosion created a narrow canyon which was moving uphill, and which eventually could have reached the spillway structure itself and swallowed a chunk of it, releasing as much as a 30-foot wall of water that the Feather River would not have been able to contain. The damaged main spillway was then reopened to lower the lake level below the point at which it over-topped the emergency spillway. The damaged main spillway is holding so far, though it will probably be so badly torn up by the additional water flowing down it now that it will have to be entirely rebuilt this summer. The powers responsible for the dam are saying that the dam itself is not in any danger of collapse, the only threat comes from the eroded hillside below the emergency spillway.
They have to keep lowering the water level in the lake as much as possible in anticipation of the next set of storms storms, and hope to have it down a considerable amount before those storms arrive. That will probably be very late Wednesday or early Thursday morning. Luckily, this set of storms will be a bit colder than the last one, so there will be a lot less snow melting in the mountains. That should reduce the amount or water flowing into the lake considerably.
But now there is not only the dam's main spillway that needs to be repaired, but the hillside below the emergency spillway will have to be patched up and made more robust somehow before next year's rains arrive. Currently, rocks are being dumped into the canyon that started to form in the hillside it, but that can only be a temporary fix, and might not be enough to make that spillway safe if the lake over-tops it again. There is going to have to be some major reconstruction going on for quite some time, and it's going to be very expensive.
It's annoying me a bit that all this is happening only a few miles away and I can't get there to look at it. The most interesting thing to happen since the big fire a few years ago, and again I'm stuck watching video on television and the Internet. Hmph. Maybe I'll see if I can find my copy of James Thurber's My Life and Hard Times, which features his classic "The Day the Dam Broke" (amusingly discussed, and with actual historical background, in this weblog post.)