1) Shipping- the upper needs to be maintained for the boat traffic coming out of the locks. That, I understand as to how much total dam is being released or kept.

2) Fossil beds at the museum- I've heard June 1 is the target date to get the upper closed ever year so all the kids, school groups, church groups.....etc that visit the museum have the chance to walk out on the fossil beds.
Plausible, but if they can or will do that "for the kids" some sauger fisherman could also use the help in the winter months!!

3) Electric demand and need for water Again, that has to do with total flow and release.

4) I would venture to guess they open the upper to help keep debris away from the hydro plant when it gets up high Plausible, good guess too.

5) Is there ever any logic down there?? . Not that I'm aware of!! Best guess you had