Get rid of the willows, particularly at the dam. Their roots will grow, then die and leave a path for water to escape. This is actually more of a problem if the trees are back of the dam.
For the bigger willows, if you can get to them, pull them out with a truck or tractor. The roots may remain and sprout, you will have to keep killing them. Someone may know of a chemical that will control them for you, but I dont. If they are growing on the bank, out of the water; get an old metal wheel off a vehicle and use it in pulling them out. Tie a chain around the tree trunk at the bottom, right next to the ground. Set the wheel next to the tree so that it will roll toward your truck when you begin pulling, and lay the chain across the top of it, in the groove, so that when you begin pulling, the wheel will make the chain pull up. The wheel will roll as the chain begins to move, and cause the force to be pulling up (instead of sideways) and pull the tree up and out of the ground.
