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.
I look at it this way:
We have a “wet area.” An upper strata of that wet area had organics to the extent that grass grew. A lower strata did not. The lower strata, like the upper strata was still within a zone we are calling a wet area.
The lower strata, when saturated, would sometimes be also exposed to rain. The rain would repeatedly carry off the “fines”, such a fine sands and smaller grained non-organics. As it did so, the larger gravels were striated into rivulets due to the action of the small grains being carried by water pushing the larger cobbles into stripes.
Another general observation:
DWR seems to have arranged an unintentional experiment for us dam watchers to observe. Apparently they plan to open the spillway when the lake level reaches 850 feet in the next few days, and then leave the spillway closed once they have again lowered the lake.
Since the snowpack is at a record level, it seems likely that we will have much higher than normal runoff. DWR apparently “thinks” the reservoir will have enough capacity to hold this much water without using the spillway.
If the DWR is right then this approach will give them more time to work.
The “experiment” comes from the lake level once again rising to a level near its full capacity. If the head of water has an impact on seepage in the green spot we will most likely see it sometime in the next few months.
I very much hope DWR will have a credible means to actively monitor and study the green spot during this period. If they believe their own propaganda (”its just rain”), then they may not.
Correct
EH: "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."
Correct
EH: "A side issue I have not seen addressed is the SIZE of these channels."
Channel widths: avg: 6.69ft, 5.72ft, merged: 10.68ft, 20.55ft, (they get wider as they combine/merge as in the two larger numbers).
EH: "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."
Correct
KCB: "The lower strata, when saturated, would sometimes be also exposed to rain. The rain would repeatedly carry off the “fines”, such a fine sands and smaller grained non-organics. As it did so, the larger gravels were striated into rivulets due to the action of the small grains being carried by water pushing the larger cobbles into stripes."
Precisely Correct