You should be working there!
I’m thinking that there’s been some water moving through the soil for quite a while, but it has been invisible due to vegetation. The evidence you show certainly suggests that there is an issue with excess water, and it could possibly be from as high up as the gates. Maybe not the gates themselves (time will tell), but the first seam where the spillway butts up against the gate structure.
Oddly, I think that there might be more water seepage at that point when the gates are closed, because the water has minimal velocity and won’t “jump the gap” (if there is a gap) between the gate structure and the spillway deck.
Hopefully, no piping. But if there is, it needs to be dealt with this summer for certain. This is high up enough in the reservoir that the water will be lowered sufficiently to work on it. The possible seepage on the dam itself is much further down in elevation, I think.
Another observation - prior to running the spillway, the plant was flowing around 12,700 cfs. Since shutting off the spillway again, it’s been running at 11,700. I wonder if that’s because of the debris removal process or if it’s for some other reason.