No limestone.
Geology there is complex. The western slope of the Sierras is a chaotic jumble resulting from two major tectonic plates pushing against each other and causing uplift. Terrain from diverse geologic eras can appear right next to each other, separated only by slip lines that can be near vertical.
I have a gold mining claim 40 miles NE of there, up a tributary perhaps 150 river miles upstream. Bedrock there is serpentine which is sedimentary layers compressed and aged into laminations that are near vertical instead of horizontal due to this uplift. Modern streams have cut down to this bedrock, and the streams carry gravel bearing gold nuggets (
we still find a few, the '49ers got most of them). This gravel is the streambed residue from a far larger prehistoric river that the modern streams intersect. (And in my area a lava ash cap from Mt Lassen, similar to concrete, overlays all this). The mines in my region generally extract this ancient streambed gravel and crush it, in contrast to a hardrock mine.
The dredger tailings hauled up the Feather River Canyon to build that dam were the output from floating gold-extracting dredges that operated up until WWII in the area below the present dam, processing ancient river gravel that deposited down there where the Feather River velocity slowed.
Its complicated. Those hillsides surrounding the dam are likely hard enough to not erode, but a perhaps greater risk is an earthquake that shifts land along a fault line. Filling the reservoir led to a couple of earthquakes. Who knows what stresses are building with the reservoir now full.
