Another variable that should be included is the phreatic saturation of the core and fill zones (time at a given level). (to which you properly brought up in a "period" notation).
Another "unseen unknown" variable is if a threshold has been crossed where there may be an increase to the defect or source that has changed from in the past.
The Division of Safety of Dams have been noting this area for years in their inspection reports - for monitoring & determining flow. Only in 2016 was a piezometer sensor (water sensor) test well drilled near this area by DWR. This should have been done long ago. There is nothing like direct evidence in getting to the root cause of something that shouldn't be occurring on an earthen dam. Any "unknown" such as this, is unacceptable as this sign are known signs of a potential failure mode.
To prove the certainty of safety requires removing the uncertainty. DWR knows there is an uncertainty as they have officially requested from FERC to place this test well at 670' in the hillside to try to figure out the cause. DWR wouldn't do this if they felt it wasn't an issue.
There are two outwardly visible characteristics of the Oroville dam “wet spot”. First, it’s “green”, indicating it generates enough water sustain plant growth. Second, there are visible vertical streaks below it, indicating at some point there was enough water percolating out for multiple channels to flow down the surface of the dam for some distance.
One hypothesis is that this flowing water originates from within the reservoir. When the reservoir level drops below the elevation of the wet spot, the wet spot dries up. Conversely, when the reservoir rises, the wet spot must get wetter. Ultimately the flow rate to and through the wet spot would have to be a function of the driving head, or reservoir water level.
Six years ago, around this date, Oroville was running close to 890’. According to DWR’s online records, the reservoir stayed in this range through mid August. No one registered much concern about it at the time. Indeed, most would consider it prudent to max out the use of the reservoir, assuming its safety was assured.
During the 3 months in 2011 that lake was at this 890’(+) stage, thousands of people drove over the dam to get to the boat ramp. One might assume these individuals would be highly motivated to notice an unusual green spot, water percolating out of the dam, or any other visible sign that the reservoir they entrust their family’s lives to was in any way impaired. This (apparently) wasn’t an issue at the time.
Even if there is a delay in water getting to the wet spot due to some hysteresis effect of pressure migrating through the dam stratums, the pressure and flow would stabilize after 3 months of constant 890’(+) head. The maximum flow rate to the wet spot would be when the head was highest for the longest period, which would have been in the summer of 2011. Even if much of this water evaporated when percolating out the SW facing slope in the daytime, the vegetative green spot should have become and stayed greener longer than has been the case in 2017, when the average lake level has been 40’ lower.
Alternate hypothesis:
It appears that the local geology consists largely of weathered brown rock and harder blue rock fractured together so as to easily form erosion channels. There is a large hill at a high elevation on the left bank. It is plausible that rainwater flowing in a ravine above the left channel sinks down into a plunge pool, through a million year old erosion channel, and into a river bank somewhere under the area where the dam got built. This either wasn’t noticed or was’t patched correctly during dam construction.
This high pressure water, injected from the hill above, would flow through the various dam fill stratums and not necessarily come out at one place or one elevation. One might assume that if a 700’ earthen dam has a maximum settlement of less than a foot (0.1%), then all the various stratums were fairly well compacted, and pressurized water might migrate between the fill layers rather than through them.
If this hypothesis is viable, then DWR’s half assed explanations may be half assed valid. The green spot is due to a local ground water “spring” or pathway that is only active when it is raining. This could help explain why some water flowing in the galleries appears to have higher organic content than might be expected from dam fill material alone.
Now DWR may not want to outright admit that spring water from the adjacent hillside occasionally flows through their dam. However, this would be an order of magnitude better than having water from the reservoir flowing through the dam, because once started, that pathway might not stop.
I think that everyone reading this forum agrees that a more through examination of this “green spot” should take place. When examining “unseen unknowns”, no plausible hypothesis should be rejected without due cause.