Excellent quiz. I knew 16/17 and by randomly guessing large countries soon got the 17th with two and a half minutes to spare. Should you consider reducing the time limit - unless you are happy with an average score rate of 15/17.
This. I did not recognize the name because I have always seen it spelled as Tisza but I looked it up and apparently it can also be spelled the other way. I suppose Tisza looks too Hungarian to be challenging and that's why it is spelled without a "z" in the quiz.
Rivers really amaze me. I didn't know the Brahmaputra (or in other cultures: Yarlung Tsangpo/Siang/Dihang River/Luit) so I looked it up and the course of the river is very interesting. The wide river on the very high plains changing into a thin (probably deep and rapid) river cutting through giant mountains ending into the great delta of Bangladesh. Everywhere having huge impact on culture, agriculture, economy, religion, transport, and so on...
Please, change Oder for Odra. In Germany the river only touches the border. Both Poles and Czechs calls it "Odra" and that is a majority. I actually missed that one, though I am Czech, because I had no idea, what Oder means.
There is also a tribuary of the Paraná River in Brazil called Rio Grande which is almost 1100 km long. I'd suggest adding a line saying that in case of multiple rivers with the same name, only the longest one counts.
And that's just one of them...