I have to admit that I am not an expert on river levels compared to draught of ships.
Although I understand that each ship must have a certain amount of water under it to float properly, I was not really aware what level is required for the Viking ships. I think that the Viking ships and other Rhein river ships have more of a flat bottom that allows them to move along the river easily at lower water levels.
I looked at the same website as you. On the web page at http://www.wetteronline.de/pegel/pegelkarte.htm
you see a list of rivers on the left hand side. The "weitere flusse" is a pull down that shows "other rivers" (the literal translation). You click on Rhein and get a listing of measuring stations. The column "Abweichung von Normal" (change from normal) is blue and I read the footnote on the chart and see that blue indicates normal pegal movements. Although you and I are using these charts to determine if there is enough water in the river, their normal purpose is probably to determine if flooding is immenent (sp?).
Not being an expert and having seen water level measuring equipment (ruler type sticks and electrical measuring devices) on the sides of the Rhein and other rivers, I thought the measurements were from the bank of the river, not the middle. Not sure now if this is true. I spent some time reviewing German websites tonight, but it is still not clear to me how the measurement is taken. I also don't know what engineering techniques are used to control the water level of the rivers.
I can tell you that I have lived in Germany for over 35 years and we are having a regular Summer, not hot, not dry. Our weather is no where near what it was last year when we had the warmest Summer in over 100 years that caused the river levels to be too low for ship traffic.
It is my laymans opinion that the river level will probably not affect a Rhein River cruise this year.
Sorry if you think that I have misled you or others, probably should be more careful before I "open my mouth".