I sort of see two options here. Spend allot of money and clean it all up, remove all the trees and rebuild the dam, or keep it like it is and make a few small, simple and cheap improvements.
A friend back had a pond that had a spillway off to the side of the dam and over the years, it had eroded down a few feet. We didn't have tractors or any practical way to move enough dirt to build it back up again. We tried with wheelbarrows, but that dirt just washed away.
Next we tried digging out a keyway type trench and filling it with sacks of readi mix. On top of that we built a wall of redi mix sacks. We just laid them on top of each other to fill in the opening and build up the sides to create a taller spillway that would keep the water off the dirt.
The opening in your pictures looks very similar to that situation. To my knowledge, that quick fix is still holding and it's been close to 15 years.
Eddie
