There certainly are crappies in the main channel of the Mississippi here in Minnesota. Around here they also tend to average larger and are heavier bodied than lake crappies. Side channels and inflowing streams can be dynamite as it warms up, but don't overlook eddies around bridge pilings, or shoreline pockets and eddies and the other current breaks or warm water sources during cold seasons. At lot of these spots are seasonal, and one has to sort that out, but the crappies are there. Here in Minneapolis one overlooked set of spots are running street drains, better if they do not run dry, but often fabulous as the drains start to run in and after a rain.