The response to the question "What is the (xxx?)?" might sound simple--find the source measurement, mouth, and source, but between those steps there are many definitions, fractals, and hydrology and, perhaps most important, national pride. This is a problem that's so complex that many hydrologists interviewed by Mental Floss explained that river length isn't even considered a reliable measure any more. What is the longest river on earth? Clicking here: https://www.freejobalert.com/longest-rivers-in-the-world/994260/ for new information.