I have just returned home from the Viking Rhine cruise scheduled from Basel, Switzerland, to Amsterdam over a period of 6 days. The cruise was shortened considerably due to low water level in the river. Upon arriving in Basel airport, after many hours flight over the Atlantic, we were taken by bus 6 hours down the river to the farthest point upstream that the ship could navigate. This meant that we missed the Strasbourg tour, Heidelberg and Rudesheim tours (including wine tasting & Music museum visit). We missed seeing all the castles, the Lorilei, and all the picturesque scenery along the upper parts of the river since the bus ride was after dark and after we were already exhausted from the long flight. Our first on-shore tour was in Cologne which was originally scheduled for day 5 of the cruise.
From information I have received, water level on the Danube is even more critically low than the Rhine. It will probably take many days of rain to bring the rivers back up to a level where ships can pass. I suggest it would be worthwhile to question Viking soon about their plans for alternative itinerary. They seem to be rather slow in volunteering any information about changes in schedules or about refunds or credit for parts of the cruise that were not completed.