The hillside underneath the overflow spillway is literally washing away. You can see downhill portions of the spillway where the waters ate away 10, 20 feet of vertical elevation. If that erosion backs all the way up to the reservoir itself, then the lining could fail (much as the main spillway did). At that point, it doesn't matter if the dam is still standing, if there is a gash right next to it lowering the effective maximum water level by several feet.
I don't know enough about the geology of the area to know how likely it is to happen. But history does show that dams can go very quickly once erosion reaches the dam/reservoir, so alarm isn't exactly unwarranted.
Why is this being down voted? The geology of the area is such that the emergency spill off can't be eroded down to the valley floor. That's why the location was selected.
I don't know enough about the geology of the area to know how likely it is to happen. But history does show that dams can go very quickly once erosion reaches the dam/reservoir, so alarm isn't exactly unwarranted.