UPDATE: This list is fairly outdated. The easiest way to check waterlevels is this: Click a river, then the Tab "Waterlevels". Or you click on the Overview and select a river :)
This year set a new record for shallow river levels. The Danube e.g. had a 200 year record, several stretches (Regensburg - Passau and upstream of Budapest) were not cruiseable. On the Rhine the low water unveiled a 1.8t air mine from World War 2. If you are concerend about the river levels, here is a list of most European RIS - River Information Systems for your own research. Please note that water levels may change frequently due to floodings and these data can only be used as an indicator. All waterlevels are metric values - as a rule of thumb you can use the following calculation:
$Draft_of_Ship >= $Waterlevel + 30 centimeters (of water under the keel)
And I would like to point out that long term predictions are nearly impossible, due to the many variables included. To mention some of them:
* How much did it snow during the winter, and when did the snow melt in spring time?
* How humid or dry is the ground -> How well buffers it rain showers?
And you have to consider this not just for one place, but for quite a large area with very different geological formations. To give you an example: the water basin of the Danube river is about 8% of the area of the US. It includes mountain regions, hilly landscapes and low land areas in Western and Central Europe. That means some risk of unforseen weather conditions will always remain.
I don't speak french, spanish or portuguese. If you could research a similar institution for the Douro, Seine, Rhone and Saone - just write the link in a comment and I will include it in this list.