Can you name the three longest rivers in every continent?
The length of a river is found by: tracing it back to its source. If the river forms from two rivers, (eg. the Blue and White niles) then the mainstem (The river with the highest discharge) counts as the continuation of that river, back to its source.
Thus, only the mainstem rivers are considered part of a river and all other tributaries are considered their own seperate rivers.
Yes, there are rivers in Antartica, as given by the oxford dictionary.
What system are you using for naming of rivers? I would take issue with several of your answers, though I agree with Africa and Europe and I plead ignorance as regards Antarctica. In Asia, the Yenisei river system is larger than both the Yellow and the Mekong. You must, of course, consider it's longest tributaries (as you have done for the Nile, for example). In Australia, the Darling is a tributary of the Murray and therefore should not be counted - same goes for the Missouri. That makes the Lachlan, the third-largest river in Australia. In N America numbers one and two are the Mississippi and the Mackenzie, respectively and in S America the Tocantins is about 300km longer than the Madeira.
I have created a new methodology (as opposed to my old one, which was a quick Google search) to decide what tributaries count as that river. To find the length of a river, I have decided that the mainstem of a river, from its source, is the length of that river. For instance, the Yenisei is larger than both the Yellow and the Mekong if you count its mainstem river, the Selenge (and the Selenge's mainstem river), to give it a length of 5539 km, according to Wikipedia. I have also decided that other tributaries that are not mainstems are different rivers. Therefore, the Darling River is a different river from the Murray as it is not the mainstem. I have decided that the Missouri is not the mainstem to the Mississippi, thus counting it as a river. This same logic applies to the Nile and the White Nile. Regarding the Tocantins, I have, after too much research, found that the Araguaia is its mainstem, thus is longer than the Madeira.
I have created a new methodology (as opposed to my old one, which was a quick Google search) to decide what tributaries count as that river. To find the length of a river, I have decided that the mainstem of a river, from its source, is the length of that river. For instance, the Yenisei is larger than both the Yellow and the Mekong if you count its mainstem river, the Selenge (and the Selenge's mainstem river), to give it a length of 5539 km, according to Wikipedia. I have also decided that other tributaries that are not mainstems are different rivers. Therefore, the Darling River is a different river from the Murray as it is not the mainstem. I have decided that the Missouri is not the mainstem to the Mississippi, thus counting it as a river. This same logic applies to the Nile and the White Nile. Regarding the Tocantins, I have, after too much research, found that the Araguaia is its mainstem, thus is longer than the Madeira.