IF you are asking about post 65, the leakage on the backside of the earthen dam would be inferred from penetration of the compacted clay wall in the middle of the dam core. If a seam or crack formed in the clay water barrier, then this could allow "piping" of water from the pressure of the higher reservoir water through this seam or crack and then out the backside of the dam embankment.
It is normal to have some "weepage" in dams. However, if the source is not accurately investigated, this creates an unknown regarding a risk factor. What is surprising is that this seems that it was not investigated.
The wide area affected could represent a volume of water that is either weeping in a thin strata line, or could be a focused piping leak that widens into a strata line on the backside embankment. The latter issue is more dangerous as a focused piping leak could cause hydraulic erosion much quicker within the clay core - triggering an escalating situation.
So far, I've been able to get sat pic evidence of this condition back to 2013.
= = copy of post in big thread:
"What’s the fix.."
The first step must be to investigate it. Weepage does occur at dams**, but what is unusual about this situation is that it is along a "seam area". If there is water leakage along a thin crack like defect (in the compacted clay water wall barrier), then the hydraulic erosion effects are spread. If the leakage source is a focused area in the compacted clay, then the hydraulic erosive potential notably escalates the risk factor.
The immediate prudent action to the existence of an unknown "risk factor" - in a proven long term existence of a "weepage" area with downhill erosion marks - is to lower the reservoir level below the elevation of the leak. This means, that to eliminate an unknown risk, the water level would be lowered to 660ft or below.
Then investigative procedures would be enacted to identify the saturation profile within the dam (probes). With this data, remedial repairs would be performed accordingly in the breaching area(s). (some dam remedial repair on notable leaks required excavation and replacement of the core layer defect using large volumes of mixed clay including Bentonite).