With the falls off the water level will drop to the lowest current leak, if any. So in theory a hole is pretty easy to find. You can also sometimes see a bump up of dampness at the waterline where the hole is. Muskrats generally dig pretty high up, like no lower than 1' below the water line. They wouldn't be expected to dig like on the bottom...but they do whatever they feel like doing. It seems like maybe you're not sure if there is a leak or the loss is from evaporation? A fairly straight sided container almost filled with water and set in the pond on blocks or whatever will show you the evaporation rate. And of course you have to figure in draw down for waterfalls, streams, filters. It doesn't sound like the new pond would do anything to solve the muskrat issue. Consider mortared rock over liner. That could be done to the current pond once the leak issue is solved.

Not much you can do in this situation. I am sure you don't want to drain the pond!!! It's sort of like 'if you build it, they will come.'

I need to see the rest of the body to tell, but my first impression is ground-hog. It has "cat whiskers" sprouting from the side of its face and nose; its ears lay back and has hair like a ground-hog. The head shape is ground-hog. A groound-hog's distance eyesight isn't the best, but it has very keen hearing and is instantly suspicious of any untoward noise it hears.

Nyac one of the two famous otter-holding-hands YouTube stars and long-time resident of the Vancouver Aquarium died on September 23, 2008. She was one of the last surviving sea otters from the 1989 Exxon Valdez oil spill. Here is a link that might be useful: Nyac: famous holding-hand otter dies