I'm with MeadowLark on this one.
What you have on top of your dam is called an "emergency overflow" which is a contingency if the overflow pipe cannot handle the flow. I would line the emergency overflow with stone and not even mess with expensive concrete.
As far as your overflow pipe, you probably should try and clean it out somehow, but if your emergency overflow can handle the flow easily I wouldn't worry to much if it can't be done.
For what's it worth I would use an "emergency overflow" any day over any kind of plumbing. (With the exception of plumbing one pond to another). I had my contractor remove the overflow pipe he put in my 3/4 acre pond as water was working around the pipe and creating a leak. Seems he "forgot" to install an anti-seep color (Probably more like he was cutting costs). Anyway, I now have an emergency overflow lined with stone that feeds into a pipe that is ABOVE GROUND before it dumps into a ditch and have not had problems since. I'm a big believer in the KISS principal. Water loves to work it's way around pipes, concrete etc.
BTW, why plant fatheads into an existing pond? IMHO you just threw good money away. They are just an expensive short lived snack in an EXISTING pond vs. a brand new one. Hopefully there weren't any rough fish mixed in with the fatheads.
