Here are some more websites that I found online about European river levels. Some are in English and some are not, but even when they are in English they can be very technical and hard to understand since without good maps or other knowledge you may not know where you need to look on a map or a table for pertinent information.
But, remember that you are looking for water levels in cm usually. The 4 newest Viking ships all draw 1.50 meters or 150cm or 5 feet. (By the way Uniworld’s newest ships draw 1.2 meters or 120cm or 4 feet.) So, you are looking for water levels. In German, this will look something like wasserstand (cm) or pegel in cm. The Hungarian site is in both English and Hungarian, again you are looking for depth in cm and the Hungarians translate this in English to “river stages.” Danube may be spelled Donau or something similar, and Rhine may be spelled Rhein. Almost all of these sites have links that will lead you to more details, like historical tables or maps, or graphs of trends. Once you figure out what you are looking for on any particular site, you will find loads of information.