Haha, interesting, so bodies of water take the article, but now Ukraine has a city named Дніпро :) Therefore, I would say Dnipro without an article should be accepted too.
Hi, because it's the name of the river. As I remember in English the most of water objects are going along with "the"
Well, I've been quite sure it applies only if we say something like "The river Thames", but not "The Thames"
Native English speaker here. Rivers and bodies of water take the article. "I crossed the Mississippi." "Have you swum in the Atlantic?", etc.
Common uses in the U.S:
1) On the river
2) On the Missouri River
3) On the lake
4) On Lake Michigan
5) On the ocean 6) On the Atlantic Ocean 7) On the mountain 8) On Mount Hood 9) On the highway 10) On Highway 101 11) On the 101
I know that Wiki is not the source you can undoubtedly refer to, but there are both variants in this article about the Thames: 'The Thames River' and 'The Thames'. https://en.wikipedia.org/wiki/River_Thames
And here is the rule of articles usage: https://www.engvid.com/english-resource/the-with-country-names-lakes-rivers/