Another page there with basic geographical questions raises an issue.
Δούναβης or Ἴστρος? (Modern and ancient). Even though the Ancient Greek is probably an "incorrect" extension of the name of the small section of the river near the settlement of ἡ Ἴστρος for the whole length of its course (such as has been done for the Potomac being the name of the river even above the Great Falls (cf.
Cohongarooton), or the name Yangtze used for the whole length of the Chang Jiang River, rather than just the part near the mouth, similar to Macau extending to the area outside the temple 媽閣廟 Maa1 Gok3 Miu6, or the name "Greek" for the Hellenes that we (mis?-)use till today), but was used by them never-the-less. I'm unsure whether the "modern" was applied knowingly or willingly, but it is something to at least be aware of.
Using the name Δούναβης is sort of an historical corrective in a way, probably based on the Latin name for the river