Occam’s razor indicates that these erosion channels (and by inference, the “green spot”) are not formed by reservoir water because the channels are not flowing when the reservoir head is the highest. The simplest explanation would be a rainwater fed “seepage channel”, just as the dam builders had noted during construction. If this is a deep channel with a small catchment basin, analogous to the sink hole that almost opened up under the emergency spillway, then it might normally generate low flow seepage to keep the green area wet in the winter and spring. But during intense rain, it could gather enough water to generate the visible erosion.
It would be useful to include an assessment of current 2017 photos in this green spot erosion assessment. Did the erosion channel rock pattern change this year? If not, that wound’t necessarily be conclusive.
This year had the greatest aggregate precipitation, but not necessarily the most intense. The most intense precipitation in the Sierras is during short duration thunderstorms. It is plausible the (hypothesized) catchment basin only fills during intense thunder showers, which then flows down and into the dam with enough head to erode the surface channels as the flow perks out. This could explain why the (hypothesized hilltop) catchment basin is small enough to be unnoticed, and why no one (apparently) has seen these dam erosion channels being formed, because the rain was so intense that they were seeking shelter.
All speculation? No doubt. That’s largely what this forum is all about.
A Q&D diagnostic that could be done would be to place various size and colors of stones in the erosion channels that would move or get washed away at different flow rates. That way it would be relatively easy to do a remote assessment of if and when the erosion channels have been reactivated. My speculative hypothesis is this it would be after thundershowers.
In any case, if there are existing piping channels through the dam that can sustain enough flow to erode its surface, and those channels ever merge with the reservoir water, then this could quickly develop into an uncontrollable situation. We all need to put and keep the pressure on DWR to resolve this issue before it resolves itself.
I stay way from his razor... seen too many people get cuts over the years....
To a layman like me, my starting point assumption was that the erosion channels under the green spot HAD to have been caused by flowing water on the surface. This would be very serious whether it came from a natural spring from the hillside flowing into the dam, or from a leak inside the dam.
A major problem with my thinking was that no one has ever seen flowing water on the surface through these numerous channels. The overall volume of water would have to be large to even show on the surface. If water HAD shown as a surface flow it would have been cause for an immediate loud alarm, and it certainly would have been documented. There is no record of that ever happening.
Apparently there is an alternative explanation that involves water seepage taking out some of the material, but leaving other (generally larger) material behind. That is what EarthResearcher333 has been researching, and provided evidence for on this thread. In spite of my common sense saying otherwise, he has convinced me.
A side issue I have not seen addressed is the SIZE of these channels. They are obvious in photographs, but I have never seen a picture with a person or other object that would put them in perspective. Given the size of the dam, my guess is that they are each a few feet wide, with an unknown depth (shallow relative to their width).
Another side issue I have not seen mentioned is the number of these channels, and the fact that they seem somewhat evenly spaced and approximately the same size. This is not what I would expect from a single point source. Likewise, as time went by, I would have expected a small number of channels to become dominant, with a noticeable difference between them.
My layman’s conclusion on this is that the source is relatively evenly spread out over the length of the green spot. This would be consistent with seepage along a construction related seam in the impermeable wall inside the dam, but NOT consistent with a natural spring along the side of the dam.
So... My personal conclusion is that common sense failed me - the channels were not caused by water flowing on the surface. It also seems unlikely that the water is from a point source.
If the water is from seepage along a construction related seam inside the dam, another question is how much head of water is needed to cause seepage to begin, and how long a lag exists between the seepage starting up and the water finding its way to the surface.
Regardless, the most important question is not the source of the water or whether it has ever flowed on the surface. The CRITICAL question is whether it is getting worse.