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When the grey rock has all of those fractures and veins
that run through it and underneath it, I don’t see how the
grey rock is very much more resistant to erosion than the
brown rock. It is still going to crumble away.
It is not a homogeneous layer of rock.

Similar to the concrete slabs on the spillway,
they did not “erode” away from the top side,
they washed out from the bottom side and fractured and crumbled away.
(IANAG - I am not a geologist)

I”m thinking, based on what I read, that the gray rock below might be a good foundation, but perhaps not a good water-flow surface, especially at 40,000++ cfs. I mean, that’s a LOT of water and thus a lot of potential to erode. I mean, it’s obviously harder and more resistant than the brown, looser rock above it, but I don’t think it’s up to the task of a high-speed water flow.

And just to note, I also am not a geologist. Nor a civil engineer.

"When the grey rock has all of those fractures and veins that run through it and underneath it, I don’t see how the grey rock is very much more resistant to erosion than the brown rock."

Here is evidence to the question:

Core Sample Analysis - A hard Amphibolite rock seam (blue grey color) observable in the middle of the longer core sample being inspected

That core sample had fractures that would force any structural design to require a consolidation of the material via high pressure grout injection. But that still would be a poor option as this sample amphibolite is fractured there is a mixture of weathered rock present. This core sample is very unusual in that the amphibolite is "sandwiched" as a laminar layer fold or seam. It is not surprising there are fractures in this narrow "seam". In addition, that core sample is not a representative of large emplacements of blue grey bedrock (amphibolite) found in large sections of the original main spillway (image below).

The image demonstrates the resiliency integrity of large sections of good fresh blue grey Amphibolite - compared to "highly fractured weathered rock". This section took quite a beating from imbedded debris within a powerful hydraulic pounding when the spillway was operating at 100,000 cfs (note: part of the flow was still coming down this section with debris, even though a greater flow was forming the "canyon"). There were chunks still coming down in this section. Yet the #11 anchor rebar is observed intact within the Amphibolite and much of the spillway is still anchored into the Amphibolite foundation. The damage to the concrete reveals the jackhammering pounding debris environment during this flow (spalling & rebar entangling). If the Amphibolite was highly fractured, you would see a significant elevation difference from erosion compared to the leftover concrete slab grade.

Bottom line: The structural conditions of the main spillway and the emergency spillway hillside are complex. Before any proposed designs are suggested, care must be exercised in thoroughly sampling the geologic substructure to determine what stripping to do, what grout consolidation is required, and what areas are too risky to construct upon.