Rumor has it the hillside where the spillways are, have a foundation of bedrock which the high velocity water flows can’t erode. So yes the area of the dam and around the dam are not eroding, so this has nothing to do with the construction or integrity of the dam itself. This has to do with the geology of the surrounding topography.
Well that and that “emergency spillway”. If that began to fail with a 30’ head of dam water behind it, the flooding would be catastrophic, but not the same as Armageddon, which is what you would get if the Dam itself failed, all 700-feet tall of it.
I used to live near there and you will find to this day huge mounds of tailings where the gold miners used pipes coming down the mountains and with high pressure water literally washed away whole hillsides. Sure there is rock around the dam but its not solid rock like you have in Colorado. You get enough water pressure against it and it will erode, its also not hard rock either for the most part.
Have seen mention that the rock is soft limestone. Anyone have hard information on this detail?