Never fished a full river flowage for crappie (only oxbows), but I have for bass and northern pike for many years. I was a student of the writings of Dan Gapan many, many years ago. He was of the notion that fish in a river system tend to stay in the eddy of the current facing upstream. Consequently, they attack their prey as is is coming downstream to them. Which kind of makes a lot of sense. He was/is an advocate of "slipping" a river - motoring upstream against the current, but not quite fast enough, so that your boat literally slips backwards, slowly. This way you can present your offerings deeper as you target those eddys.
Not sure how this would work for crappie fishing, but it might have some possibilities.
"A voyage in search of knowledge need never abandon the spirit of adventure."