We boat and ski in the Mississippi and Illinois rivers regularly. I do have the card with the detailed maps that
@airshot mentioned, but even with those charts we still have to survey the section of river before putting anyone behind the boat. Too may obstacles - floaters, entire trees lodged in the river bottom, etc. - to trust that the water is safe. We just cruise the section of river once or twice to be sure everything is clear.
One feature that you might find helpful in a GPS/fishfinder unit is a 'breadcrumb' feature that shows the path where you previously have boated. Our GPS has information on our last several outings, so if you often boat in a particular area, you'll have a record of where you were. That can help you identify safe parts of the channel, places to beach, etc.