[eck]

People have been saying that if the emergency spillway fails, it will release the top 30' (?) of the lake. What determines that height? Is there bedrock or something else expected to stop erosion 30' below the level of the emergency spillway or something?

[mikeash]

Good question! According to this article, the bottom of the emergency spillway is indeed bedrock: http://www.drroyspencer.com/2017/02/why-the-oroville-dam-won...

[Obi_Juan_Kenobi]

Yes, it's basically on bedrock.

This image of the main spillway gives you an idea of what they're building on: http://i.dailymail.co.uk/i/pix/2017/02/13/08/3D252AE20000057...

You can see that they blasted into the rock to make the spillway.

However, the idea that erosion will undercut the emergency spillway and cause it fail implies that it's not built directly on the rock. This suggests that there could be more than 30' of draining.

[khuey]

The emergency spillway is a 30 ft concrete wall in the side of the lake.

[WildUtah]

If you look at the photos, some on Wikipedia, you can see the spillways are around a bend in the river from the main dam and pass over a high bedrock canyon wall.