They don't know. DWR even asked FERC in 2015 to allow them to relocate a test drill hole (put in in 2016) with a water sensing piezometer up by the abutment near the Seepage area to help them try to figure this out.
More alarmingly, a DSSMRs report identified a 2x settlement difference on the dam. DWR was asked by the Board to get an answer to this 2x differential settlement and respond with an updated DSSMRs report to FERC.
This 2x finding is a significant sign of "differential settlement" issues. Differential Settlement are Dam killers.
The Greening Seepage area is in a classic "sharp abutment" transition from a flat abutment zone. Stress forces from this characteristic are warned in Earthen Dam designs as a failure mechanism as "differential settlement" will occur at this transition. Thus, potentially large differential compaction differences will create longitudinal cracks in the core - causing leakage.
Based on your assessment what could you reasonably predict as the time-frames of a worst case and best case scenario (IOW how much time do the residents of Oroville & environs have - worst case/best case)?