Probably not the best person to answer your question, but thinking out loud, I would imagine how the dock relates to flow (in the river) is more important, crappie like easy meals in typical slower/eddy water...hopefully a seasoned dock shooting veteran can blow up my theory & answer better, do know certain docks are better than others, so many variables, like closer to the water one's cast bigger shadows & keep them out of the sun....good luck & report back if you find a pattern.