I catch some runty channels in the Mississippi between St. Cloud and Little Falls. They top out at 5-8 lbs, with a 10-15lb in the mix at times. I have seen them over 20lbs taken from below the St. Cloud dam, but I have never caught one that big myself.
Here are a couple "average eaters" from below the Royalton hydrodam...
Ir you are looking for some real action, find a guide service on the Minnesota River. I have heard that the flathead population is doing fine and they are catching some monsters even in the daytime.
Here is a quick read for you..
http://www.brainerd.com/rec/cats.html