Before they shut off the water, it eroded out the left side of the spillway and was simply washing down the hillside.
But the emergency spillway, shown here in red, has never been used and would simply spill down the hillside, eroding a big trench and even undermining the spillway itself. That might be a significant problem for everyone downstream.
Here's the top of the emergency spillway, which looks from this angle like it dumps the water right on the ground downhill - a recipe for erosion. If that happens, then it could conceivably undermine things, cause the concrete berm here to collapse, and let even more of the lake pour over. Bad. Yuge.
The only safe thing to do is simply dump water down this damaged spillway, knowing that you will lose everything down hill. Better that than letting the river overtop the emergency spill way and gamble that that never before used tool works as planned.
I'll be an expensive fix this summer, but still much less risky than the other options. Let's see what the experts actually do.