https://www.youtube.com/watch?v=wTyCPPqv5pc
Here is another video showing the main spillway with the dam in the background. From other photos I’ve seen it looked like the spillway was on rock. This video shows part of it is, but it looks like a lot of soil/weak rock is being washed away between the spillway and the toe of the dam.
If too much material gets washed out below the toe, the dam could go. Yeouch.
If that one is the same as this one,
https://www.periscope.tv/w/1mrGmeVBkqdGy
you can see a chunk of the hill to the right eroded away... A swath 3x the width of the original concrete spillway.
This view shows more... H/t maggief
https://www.periscope.tv/w/1mrGmeVBkqdGy
A swath 3x the width of the spillway (the one that had the sinkhole in it) has been cut by the water overflow.
It’s to the right, toward the dam.
You can see big @$$ chunks of concrete tumbling down the original spillway..