Al had two business associates here the last few days who are from Germany. This drought has been terrible for businesses that rely on transferring material on barges. Many barges have been unable to get down the Rhine to their destinations and have had to turn around and go back due to shallow water. As Jim said, they have had no rain yet, although they keep predicting it will come.
Per cruise critic, AMA Waterways has come up with any interesting solution. Ships heading down the Rhine are docking before they get to the shallow areas. Ships heading in the other direction are doing the same. They are then having passengers switch boats after being transferred by busses. The crews are staying with their respective boats. There have also been cruises cancelled with about one week's notice. Either way this is not a good situation, especially with non-refundable air and pre-paid hotels.