Rainbow trout are stocked in many popular streams and area ponds a crossed Wisconsin. But very few of them are reported to have naturally occurring populations of Rainbow Trout. The species is a native of the west coast and far eastern Asia. Is some areas of the American West the Rainbow has negatively impacted native fish species by out competing native fish, transmitting contagious diseases, like whirling disease or hybridization a good example of this would be the Yellowstone cutthroat trout.
But here in Wisconsin there are only a few places where someone would be able to catch a wild rainbow. Most tributaries to Lake Superior have naturally reproducing "steelheads" where the smolt will live in the streams for 2-3 summers before moving into the lake. At this age they are typically around 6-8 inches in length (although this depends on the stream) Although not all trout will move into the lake, but some will live their entire life in the stream.
Moving inland..................... Black Earth Creek and Mount Vernon Creek in Southwestern Dane county have SMALL populations of wild rainbows and I have caught a handful over the years.
If you really want to catch a wild, stream born rainbow trout in Wisconsin look to Central Wisconsin in a few counties like Waushara county and the surrounding areas. I have found a great place to catch wild rainbows in Northeastern WI.
Rainbow trout prefer 55-60 degree water with a faster current and a gravel or rocky bottom. They are often found at the head of rapids or strong riffles.