Well, the upper Missouri valley has been in a drought situation for a while now. So the biggest feeder into the Mississippi is way down at the moment.
the Ohio's pretty strong, especially with the Tennessee and Cumberland rivers flowing high at the west end. Lots of rain came up through western Tennessee, Kentucky, and up into Ohio before it worked its way out of the Mississippi watershed. That'll work its way down to NOLA soon enough. Hopkinsville, KY was flooded this morning, after having endured rain for the last couple of days from a stalled cold front that was being drawn together with Katrina.
I can't say that it's a huge total amount of water, but the Mississippi will flow a bit above average for a couple of days, IMHO.