This quiz might need a couple of revisions before it is finished. There is no single source with good data for this, so I did some original research that involved a certain amount of guessing. Please let me know if there are any glaring omissions or mistakes.
Manaus; Belém; Macapá; Parintins; Coari; Tefé; Santarém; Porto Velho; Itacoatiara; Manacapuru; Tabatinga... but they aren't too much big (and the majority is in Brazil), except Manaus and Belém. Well, the biggest is not always the biggest...
The Edo is less than 60 km long and only passes through one metropolitan area. It's a massive metro area to be sure, but citypopulation.de still has it at less than 40 million.
Really thought the Zambezi could make it, as the basin is quite large. Worked it out and get to 42.7 million. Most of Zambia (83% = 13.8), Zimbabwe (57% = 9.2) and Malawi (83% = 15). Some realistic/conservative numbers for Angola (Moxoco+ = 0.7), Botswana (Chobe+ 0.1), Namibia (Zambezi 0.1), Tanzania (Distr Ileje,Kyela, Rungwe, Makete, Ludewa and Mbina - est. 0.7) and Mozambique (Tete+ = 3.1) and it seems the 50 million is a stretch. Province and district by district calculated to work out the % and population estimates. Might be useful info for another quiz though. :)
Please accept 'Yangzi' for Yangtze. Wikipedia reads 'Yangtze or Yangzi'. Also, pinyin, the most common transliteration of Chinese, uses 'z' not 'tz' for that consonant.
Huai River should be added in this quiz. There were over 170M people living in its basin in 2005 -- even without counting the population along the North Jiangsu Main Irrigation Canal, which is its main passage into the ocean.
It's true that during recent years the Huai River basin has experienced some outflow of pupulation, but it should still be well beyond 150M.
https://en.wikipedia.org/wiki/Zambezi#/media/File:Zambezi_river_basin-en.svg
It's true that during recent years the Huai River basin has experienced some outflow of pupulation, but it should still be well beyond 150M.