Elbe and Vistula come to mind before either of those.Anyway, out of more significant European rivers, Tagus, Po, Rhone and Loire come to mind immediately.
Daugava too perhaps. But yes, I agree that those four should go in as well. And maybe Seine and Thames, while not as large, for their historical significance. But, like Symphony has pointed out, it's an issue of finding the proper data for them.
