[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.