This shoreline damage is caused by muskrats. Muskrats live in the muskrats huts that people are used to seeing in large marshes. However in North East Wisconsin, often muskrats burrow into banks of ponds, stream, and lakes.
These burrow often become hazards for caving in when walking or mowing around the shorelines.
Please contact Suburban Wildlife Solutions for all your wildlife answers at SuburbanWS@Gmail.com