Hi, these forums seem pretty dead, or maybe no one lives south of the cities, anyways i can give you a little help on that but not much. I live south of Cannon falls in Northfield and joke around here is you dont throw anything in the water or you might give one of the fish with 3 eyes and 2 legs a black eye 
but other then the pollution i remember once seeing a kid hauling up carp off the bridge square Dam. He caught so many he started handing them out to people. But i have fished some of the fast waters south of Northfield near Dundas and spent a better part of a afternoon fishing and didnt see anything. Of course i wasnt fly fishing so your results may be different. But it is local knowledge around here that you dont want to drink that water or eat anything out of the cannon, although fishing wise i cant really speak for it all, but i know around our way of cannon falls its So-So bait fishing, and ive never seen anyone fly fishing it. Although there is a place, its kind of a mystery because its got so many variations on its name, its called Spring brook creek,and its suppose to be one of the few places there are trout around here. Ive only seen a couple referrences to it on the internet and it kinda shows up in different places, all of which are owned by cow farmers, and one fella i know who speaks like the godfather and probably just as quickly get rid of you. So if you got good insurance you could look for that place . Sorry i cant be of more help but hope that leads you in the right direction, as in closer to the mississippi!