I see many good comments here. My suggestion is to have the bottom property line come out just above the letter M at the damn. Have the entire land under the damn his. Widen your red section across the road to offset it. I would also include all of the drainage below the damn on his part of the property. You still can access the lake where it encroaches on your land. The entire maintenance and liability for the damn is 100% on his side. Yes, he could remove said damn in the future. But you have gained fully another piece of property. You might want a second small piece of land on the property directly across the road so you could have your son or anyone build there. Thus, I would not just do a lot line adjustment to the new boundaries but if you want a lot line adjustment do it only on the land you are giving up for the damn and below the damn. You would have part of your original piece shrink and gain a total new second piece. You might consider offering 12k toward the second piece and the agreement clearly states his is 100% responsible to build and maintain the damn, but you have access rights as an addendum to your deed. I think the damn should have a spillway. I also think there should be a built in gate to allowing draining. If in the future you get some nuisance animals (the birds mentioned above) then just let it fully drain down. A few months after nesting time you could close the drain and the spring and rain run off would fill it back up. Looks like this could be a good win win.
