Thank you, your response certainly makes sense. My concern was/is that even with significant rain events I am not observing the syphon flow that we experienced on previous occasions. In that we are not seeing overflow out the spillway, or any indication that the water level is changing by an appreciable amount I guess it is best to leave well enough alone.

Your explanation also makes sense in that when we capped the outflow the syphon DID kick in and work as designed.

We are scheduled to electro-fish this Friday if the rain lets up so not worrying about the drain is a real relief. I do appreciate everyone's help and advice. Thank again.