I had the same issue with the drain pipes for both of the ponds on the property I recently bought. I was able to unclog both of them by connecting sections of 3/4" pvc pipe ~ 80' total. At the head end of the pipe I drilled a 5/32" inch hole through both walls of the pipe and inserted a length of 5/32" tig wire. I bent the wire to parallel the pipe and then bent 2" "barbs" into the wire. I also attached some small rope just behind the wire and added 3 half hitches and secured the rope and the wire with duct tape. The rope was used to pull the pipe from the first joint in case it got stuck (which it did). I didn't want the pipe to come apart inside the drain pipe and make things worse. I then pushed the pipe in as far as it would go and gave it a twisting and pulling motion. I had them both cleared after about 5 minutes of work. This worked great for me, hopefully it can help in your situation.
