We're scheduled to take the 8/7 Amsterdam-Basel cruise. I called Viking's 800# and was told the following:
Some rivers are impassable due to low water levels. As a result, passengers are/will be given the option of continuing the vacation using buses/hotels and joining the ship when/if the river level allows or returning to the U.S. and receiving some form of refund/credit. For current passengers, such as yourself, the choices may be more limited as Viking claims they "just discovered" the water level problem.
They claim they are providing more advance notification of at least several days to other affected passengers (e.g., those who have not left the U.S.) and offering them the options described above to include canceling the trip. The woman to whom I spoke indicated that the Basel-Amsterdam cruise that is set to begin tomorrow (I think) is still on. However, the water level situation can fluctuate daily, for better or worse.
The good news (at least for the Rhine/Moselle route) is that rain is forecast off & on for the next week, which may help the water levels. Viking indicated that passengers who have not yet left for Europe can call the line in advance of their trip to get an updated status. However, as a practical matter, the line probably won't know more than several days in advance what the situation will be and, as noted, even then it can change quickly.
That's all I know.