Here is the earthen berm between two production ponds where a muskrat bore through. Distance of the tunnel is at least 10 feet. Note the shovel which marks the entry point and downward angle of the muskrat hole. Also note a temporary fix on a washout on the other pond with the blue tarp. I need to get a load of limestone on to that spot but it hasn't happened yet. Also note the two PVC pipes that are used to fill the ponds from the well. The one on the right has a length of 6 inch drainage pipe slipped over the 4 inch PVC. This prevents gouging of the pond bottom when the pond is filled.
The spade shovel handle shows the how deep the slot I dug through the tunnel is (about 3 feet) A spade shovel is great for digging a slot in clay without digging more than necessary. It was cut close to the middle of the earthen pond petition (I have a pipe to the trout pond in the center so I didn't want to dig into that). Once I found the muskrat hole on each side of the slot I dug about 8 inches deeper. This beats just plugging the hole from one or both ends and also beats digging up the entire tunnel for obvious reasons.
The slot will be all or mostly filled with concrete mix after I plug the muskrat hole from both ends. This is a similar principal to a putting a collar on a pipe in a trench. Even though I pack the holes from both sides with wet clay and a pole, in case water gets through it will have to stop at the concrete collar. (Says this with fingers crossed).
I will spray the cattails (which are a food source for muskrats ASAP (low on funds right now for the herbicide and surfactant), and when they turn yellow they will all be cut out. Also the ponds will be completely drained each fall and the production fish moved. Hopefully this will prevent any future muskrat problems. I've never had a problem in the trout pond that has a thick layer of landscaper size stone down several feet into the water column.
Anyway that's my method. Feedback is welcome.