We have one pond that has a 15 inch sewer type pipe through it as the principle spillway. It is about 150 yds in length, but ends short of the normal draw channel. It only runs in the spring and after a heavy rain, but what I did to control erosion from the end of the pipe down to the normal draw channel was to make a field stone "stream" about 10 ft wide. We shaped the channel and then starting at the bottom end, layed sections of heavy gauge plastic, over lapping as we went up, much like the shingles on a roof. This plastic was actually just "ag bag" plastic I got for free from a farmer who was using it to store silage the winter before. Then we laid field stone in the channel on top of the plastic. Every 15 ft or so we made a sort of small waterfall by placing some big rocks across the channel. These also serve as anchor points so in a heavy flow the smaller rocks just upstream will stay put. Makes a great sound, and the water can't erode the soil underneath. Sounds like you've already got rock in place, so you may not want to go to all the work of removing them and then doing this. Just keep an eye on the soils underneath for erosion.
