As usual, excellent information and analysis.
Key unasked question: Is there currently detectable water flowing from the dense green area through the easily visible erosion channels?
If so, how much?
If not, was there when the lake was at a higher level than it is right now?
It seems it must be one or the other, or both. Otherwise, how did the multiple obvious erosion channels form? They clearly originate in the green area, they are not visible elsewhere on the dam, and it was NOT rain.
The green area is growing, which means the water flow is increasing.
So... HOW MUCH WATER?
Presumably the DWR knows the answer and has chosen not to tell us. It’s even worse if they DON’T.
A very good question. These large erosion channels are a physical displacement of fill material that is designed to have water percolate & penetrate into this zone 3 fill (sands, gravels, cobbles, boulders). So the fact that there are visible surface erosion channels in such material means that there is more water flowing that is not observable at the surface.
That is precisely why the 2014/2015 Division of Safety of Dams (DSOD) Inspection report noted that this leakage should be monitored (yet no instrumentation for monitoring is noted as emplaced over the years - other than cutting vegetation to visually observe for greening "area" changes). Deeper penetration & erosion below the surface can cause these channels to be formed by subsidence erosion. So the outside view of the channels may look somewhat dry, but the waterflow is real and its underneath.
To determine the full waterflow volume requires understanding the curvature and the extent of the undersurface erosion channel percolation dispersion passages.
This same effect is how the "greening" may vary on supplemental hydration of the hillside adding to the effect of the strength of the internal reservoir head pressure, the internal core leakage volume, and the net percolation saturation of these under surface dispersion channels.
Instead, DWR drills a test well in the hill thinking a "spring" or maybe "piping around the dam" is somehow penetrating the left abutment zone 3 fill and then "flowing uphill" (greenage pattern elevation slope away from the abutment). Yet DWR is not saying this to the public. They hold town hall meetings and say "rain falls...then grass grows".
The only way to assess an accurate risk is to quickly get answers. Assuming DWR's latest press release is what they really believe, it likely will take external engineering criticism from reputable sources before they reluctantly may take action.
Worst case, if there is a sudden change of events were water is observed to flow on the surface of the erosion channels, that would be a bad sign.