Buck,
Is the vertical tube supposed to be functioning as your principal spillway?
If it is, then the dam has been living on borrowed time if the principle spillway has been plugged. This would force all the overflow through the secondary spillway (which is usually an earthen, flat bottomed trench around the outside of the dam. The secondary spillway is normally vegetated with a sod forming grass, and is meant to be used only in conditions of 25+ year rainfall events. If the normal trickle of water that needs to go through the dam is routed through the emergency spillway, over several months it will start eroding the spillway from the bottom edge up, and eventually into the dam itself. Then when a heavy rain comes along, the increased flow can eat through the dam and cause it to fail.
If that vertical tube is supposed to be your principle spillway, then you had better try to fix it (if you can lower the water level enough), or plug it and construct another principle spillway that wouldn't vertically drop down so far. That way you might be able to work above the current water level. In either event, you'll want to find a reputable contractor that has had experience in installing these pipes. Proper placement and compaction around the tubes is very critical.
If you've got access to field stone, you might want to cover the face of your dam and below with that, after you get the burrows fixed. Rocks will help deter the rats you don't get (and you'll never get them all).
Bill
