re: Pond is leaking underneath concrete damn/weir

build a wooden wall like structure with plywood and posts just about a foot in front of the old dam, throw tarp in front and over it, then add sand and dirt on water side to seal off water so the old dam is dry, then make any needed repairs and repour concrete higher then it was before, as well as in front of the old dam

after you can remove the fence and tarp or just leave it there to rot away to nothing