[Naritai]

You have to remember that the hill _is_ the dam (of the type[1]). so when water flows down that spillway, it's going to pick up dirt with it - making the hill smaller and smaller, ie. making the dam thinner and thinner.

[1]https://en.wikipedia.org/wiki/Embankment_dam

[ChefDenominator]

The dam is the earth type of dam, but the portion the spillway is on is not part of the dam, at least as I am looking at it in Google Maps:

https://www.google.com/maps/@39.5400137,-121.4931958,3617m/d...

[anotheryou]

But if it eats up the spillway upwards it engangers the little part of tho damm where the vales are. If you do not have a concrete lip like on the auxillary spillway and it just flushes over the top of the hill it will quickly carve a lot deeper, so you have more water than just what was held back by the smaller wall.

Also: the top 70 meters or something is still a lot of water!

[Naritai]

You're drawing distinction between the 'man made' part of the dam and the 'natural' dam, which exists on paper but not in the static force analysis. If that natural portion of the dam becomes unable to support the weight of the water, there will be a collapse.