My friend and I built a duck pond leveling feature in an old swamp pond, long forgotten by the people who had drained it long ago. We put the structure in the old ditch dug to drain it. The beavers had clogged it a few times, but had abandoned it when we got to it. So, we put in our stuff and planted for duck hunting. We put the plug in and start getting some water in there and the muskrats moved in within a few weeks of getting a foot of water across the whole thing.
Well, being fall, as often happens, it started raining more often and we were excited to see what our pond would look like at full pool. Apparently, muskrats don't like when they build a house and it starts getting more water in it; they must not like to rebuild.
These muskrats promptly went down to the dam and dug holes all the way through it so it wouldn't flood their houses anymore. I had a few choice words for those muskrats which cannot be repeated here. As luck would have it, I came across another friend who was tearing down some old dog fencing a few weeks after my muskrat problem started. I took his trash fence and took it down to my dam. I filled all the holes then unrolled all that old wire fence over the dam and stapled it down so they could not give me anymore trouble. it actually worked well.