There are still some small mounds in some state parks ( i.e. Perrot state park near Trempealeau, Wi). But near McGregor, Iowa is Effigy Mounds Nat. Monument where there are larger mounds.
In my teens, I worked for a farmer in the town of Ashford, WI (Fond du Lac County). He explained how he had been approached by a tribe who’s mounds had been built on the land his farm enveloped. As they explained what they were looking for, he began to remember/recognize land features that they were describing (from his childhood exploring on the family farm), and was able to take them directly to the these land features. There were several mounds, in formation, spread out over 300 acres. The mounds had been somewhat preserved because the mounds would have been difficult to cultivate, and were surrounded by stands of large trees. There was a network of trails that the farmer had thought were deer trails, but were also a part of the mound structures.
I have to go visit him and his wife again!