Actually, the water levels as reported on the internet look pretty good. You can see an actual live picture of the Rhine River at Cologne, Germany at http://www.anrheiner.de/html/10_anrheiner_webcam/
and it doesn't look low.
Have a wonderful time. We are heading the opposite direction on the River Harmony (Grand Circle) departing Antwerp on Sept. 23rd. Really looking forward to this trip.