Rainbows are a stocked species in southeast Minnesota, typically stocked for two reasons. The largest is for put and take reasons. They are typically stocked before the opener in April, Memorial Day, July 4th, and then usually over Labor Day weekend. If you want to catch them, go to a stream without any special regulations. They aren't stocked on special regs waters (for the most part), and sweet little spring creeks like Gribben don't get them as the browns and brookies have a thriving population.
Hope this makes sense,
D.A.