Update:

I've dug down about 3 feet or more to investigate and initiate repairs. The smaller holes terminated into a larger hole which is big enough for a muskrat.

Perhaps Essup is right in that the smaller holes are baby muskrats? Or perhaps I am dealing with both muskrats and crayfish? Maybe the muskrats are going after the crayfish or the crayfish are utilizing the abandoned muskrat holes? Smaller holes are air holes?

Regardless they will have to get through a concrete barrier in two places soon, which will be at least a foot deeper than their tunnels. I am also declaring war on their food sources especially the cattails. Got some Avocet and cide kick in today.

At least they didn't bore from the shoreline to the base of the dike! All holes begin at least 15 feet away from the shoreline and originate in the emergency overflow which was flowing steadily due trout pond running into this pond. This will not happen this year I am bypassing the trout water after the receiving pond refills.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.