Hello. I am a solicitor with 20 years experience. I will try to help you with this.
You think the stream marks the boundary? If you are right and the fence is in you land then it s nothing to do with your neighbour and it is up to you to keep it in repair if you want to. Your first step is to ask your neighbour why he wants to repair the fence that as far as you are concerned is yours and on your land.