Thanks for the responses -- I mean, it looks like the line runs through the dam. The GIS information shows it running through the dam. I'm trying to round up the worst-case scenarios. So far, there are two:

1. He takes out his part of the dam. Can't I just reinforce my side?
2. He says it needs to be 20 feet (if there's a law -- and it's Illinois) from the line. Can't I just fill it in on one side and expand it on the other?

I can also offer to buy some of the surrounding land. He does want to sell.

Is there anything worse than these scenarios?

I just want to get back to enjoying my pond!