I think you're on to something with the RES wanting to travel.
My pond doesn't see the massive flow-through events that yours has, but it did spend a good deal of time last spring running over the spillway.
I picked up hundreds of <3" sunfish from the grass in the drain swale. 90% of them were RES.
This discussion has reminded me that I need to put a hardware cloth fence across the spillway before the next round of rains get here.
