To get to the next clue, it helps to see how the dam was designed with respect to water retention and the natural "seepage" through the core and transition zones. The internal water conditions are noted as "phreatic surface" or "phreatic line". This is essentially the water level within the inner sections of the dam.
The dam holds back water from a highly compacted "core" layer of clay-clayey mix (zone 1 - orange in illustration). It was placed in 10 inch lifts, carefully hydrated to a certain percentage ratio, and then compacted with a form of an additional consolidation. Documents state that the net core compaction/consolidation density rivaled or bettered the density of concrete. To structurally protect and to hydraulically protect the "core" layer, "transition zones" (Zone 2) were built on each side of the central core (dark brown in illustration). The transition zones had silts, sands, gravels, cobbles, and boulders (to 15" max). The silts in conjunction with sands formed a semi-block of waterflow. The cobbles, and boulders provided the structural support that the smaller silts sands gravels were confined within. The upstream Transition zone was designed to "heal" the core in case micro defects develop. The seepage into these defects would transport "silts" from the Transition zone and plug the micro defects in the core.
The outer layers are the massive gravity weight layers of the Zone 3 fill comprised of sands, gravels, cobbles, and boulders (to 24" max). The Zone 3 fill "shell layers" are intended to pass water, thus they are called non-pervious. Although waterflow was restricted to a slowed percolation through the smaller fines in the gravels and sand.
A very important Zone layer is the "Drain Zone" (light blue in illustration). The Drain Zone is intended to direct and pass any waterflow that seeps past the transition zones, and the core. As the Drain Zone is comprised of gravels, cobbles, and boulders it has no silts, sand, or clay-clayey material to impede waterflow. The vertical section is 20 feet thick for the vertical "chimney" part of the Drain Zone, and 10 feet thick for the bottom of the dam Drain Zone layer. The dark blue arrows depict the ideal operation of all of these fill zones working as designed. There should be no reservoir waterflow penetration beyond the Drain Zone Chimney in an ideal "phreatic level" or "phreatic surface" curve inside the dam.
On the back side of the dam, the Zone 3 outer shell will respond to rainfall in allowing the rain to percolate downward into the sands, gravels, cobbles, and boulders. This design of percolation is to prevent erosion channels from developing on the outer surface of the dam. Everything works fine as the flow and the construction compaction are in unison to gravity & gradual settlement. There will be minor erosion on the surface, but this is a very long and slow process that keeps the effect retained through the ability of the material to allow rain water to percolate into the consistent strata of sands and gravels.
The two combined dangers that may "upset" this internal percolation design function are (1) high rates of percolation flow (creating saturation pressures) in conjunction with (2) a non-unison vector flow (i.e. a lateral type of pressured flow from "deflection or redirection" instead of the vertical unison alignment).
At this point is where the clues of the Erosion channels start to emerge from of a new "source" of an "unexpected type" of water flow, other than the ideal phreatic level flow, is introduced.
A whole series of internal events start to develop. More later…
