We fish the Connecticut River system here in MA, which comes down from the Canadian border lakes. We mainly target oxbow lakes and back waters on bends. I've always been told that you can catch crappie in the main river basin, but as of yet, no one has told me how!?
If your river doesn't have these type of backwaters or large eddys, you might want to consider calling your Dept. of Natural Resources and find out from them if any crappie do exist in the main river system. Maybe you're just chasing ghosts? If they tell you that there are crappie in the river, then I would also like to know how to target them.
"A voyage in search of knowledge need never abandon the spirit of adventure."