Since all of the poor foundation material (clay & highly weathered rock) is removed to clean bedrock, any seepage from sources under the final design will find its way to the drain rock channels without concerns of subsurface erosion. They will likely do some sealing of the rock at the spring before placing leveling concrete on top of it (final construction - RCC will be temporary construction there in 2017/2018 rainy season).
Even without the existence of a spring, there is water that will percolate to the surface of the bedrock to the foundation & bedrock boundary. The tremendous load pressure of spillway water flows will induce a significant "pressure force" on the bedrock. This too influences subsurface water percolation through micro cracks in the bedrock. The drains will collect this seepage from below as the graveled "channels" go directly to the bedrock.
Note: There are no formal drain channels in temporary RCC section. Thus any subsurface seepage will migrate however it can within the bedrock cracks and if any porosity within the RCC bottom seam.
Operating procedures require a gradual increase and decrease in the spillway flows to allow a gradual adjustment of the bedrock strain (change of shape) in base stresses (load force) from these tremendous load fluctuations. That is why it takes time to ramp up and ramp down (normal operations) of the spillway flows. Only in an emergency will this procedure be deviated from. These forces will influence subsurface seepage if water is present.
If I were a 4-5th grade school teacher, I'd use this thread with its stunning photographs to teach the lesson on “Conservation”, not 1960’s version that our public schools taught, but 2017 version! There are lessons in cause and effect, dam construction, moral vs. practical, work ethic, taking responsibility vs. passing the buck, the longterm consequence of evasion or omitting unpleasant details, etc. If anyone can think of more lessons for 5th classes, let me know.