As I alluded to in an earlier posting, we sailed the 17-day Eastern European Odyssey along the Danube from Vienna to the Black Sea and back to Vienna, with an embarkation date of June 24th and disembarkation July 11th--exactly the time frame you mention in your positing. We were fortunate in that although it had not rained for several days--perhaps even weeks--our cruise itinerary was not altered except for one detour of about 100 miles somewhere along the way. However, we met people from at least three other cruiselines (Grand Circle, Deilmann, and a European-based line whose name I can't recall), and their itineraries had needed substantial changes because of the river level.
Here's my question: Should Viking have cancelled its cruise because other cruiselines were having trouble with water levels? I don't think so. The thrust of any particular boat could make a difference on whether or not it can get through a shallow area. A difference of an hour or two can determine success or failure to get through a particular section of the Danube, the Elbe, and other European rivers.
There's no easy answer...