The navigable rivers are limited to ones that the river-navigating cultures are confirmed to have used in their raids/invasions/settlements. There are AFAIK no records of Norse raiders sailing up the Danube or any Spanish or Italian rivers, while numerous accounts of French, Russian, and German river-raiding.
Some maps did claim the Vikings were at the Danube. Maybe I can find more sources on this issue. Remember that many areas around the Danube don't have as much sources as Germany or France.
http://terencewilson.website/vikings-voyages.png
https://www.academia.edu/16720256/T...nges._New_archaeological_data_on_the_weaponry