I also have an "earthen" spillway that has is about 10ft wide and filled with rocks on geotextile, with a 4:1 slope heading into the spillway. With that setup I have lost many minnows and I imagine I lose some fry, but using a range of rock sizes and a few feet of shallow rocky area I have not lost any adult fish, which would be caught in downstream pools. I personally like this better than the fence since any leaves or sticks get washed over the rocks and don't fully plug the spillway, if it gets bad. I also control the inlet flow to the pond by diverting excess flow and have not seen more than a few inches of water in the spillway. Not 100% guaranteed but seems to work.
