Finny,
Sorry to hear about your nasty erosion problem. Yes the propagation of headcuts is kind of amazing to watch as the damage just keeps migrating.
I assume water is currently running into your pond at a rate greater than your normal pond outlet can discharge the water?
To give a sense of scale, what is the size of your normal pond outlet? What percentage of your current flow is going through your emergency spillway? What is the surface acreage and average depth of your pond? How deep is the fall of the headcut? (The height from the flat spot in the crest of your emergency spillway to the flat area below where the waterfall turns back into just a stream.)
There are lots of possible solutions. Let's get some more info and figure out a good short-term band-aid fix and then a longer term economical solution.
