I think you will be okay on a European river cruise this year. I have been concerned also, because we are cruising on the Rhine next week. I have heard back from folks who live in Europe or have recently cruised the European waterways. They all say river levels are fine--the season has been cool and wet.
I have followed the river levels on the wetteronline website that Ann mentions in the first post. I have been reassured in the last few days because the water level measurements on the Rhine have risen significantly. I also see on the weather maps that it has indeed been raining. Of course, there are no guarantees that the river will not fall, or rise, for that matter. We cruised the Danube 2 years ago and had a great trip. Within days after we cruised, the Danube flooded and cruises were cancelled.
Sorry to have to agree with the observations of others from last year. Viking continues to be unresponsive to customers' questions and concerns. In fact, even this year, when all appears to be well with their itineraries, I did not receive an answer to a question posed by email, not even an acknowledgement of the query.
The good news is that everyone I have heard from has spoken highly of their river cruise experience with Viking. Keep that in mind, and enjoy your trip!