^ The Nile is usually said to be the longest river in the world, with a length of about 6,650 km,^[7] and the Amazon the second longest, with a length of at least 6,400 km.^[8] In recent decades debate has intensified over the true source and therefore the length of the Amazon River.^[9]^[10] Brazilian and Peruvian Studies in 2007 and 2008 added the waterway from the Amazon's southern outlet through tidal canals and the Pará estuary of the Tocantins and then concluded that the Amazon has a length of 6,992 km and was longer than the Nile, whose length was calculated as 6,853 km.^[11] A peer-reviewed article, published in 2009, states a length of 7,088 km for the Nile and 6,575 km for the Amazon, measured by using a combination of satellite image analysis and field investigations to the source regions.^[2] Therefore, as of 2018 the length of both rivers remains open to interpretation and continued debate.^[8]^[1] Note that the disputed values have been put in parentheses.
^ The length of the Nile is usually said to be 6,650 km^[7], but published values lie anywhere between 5,499 km and 7,088 km^[2]. The length measurements of many rivers are only approximations and differ from each other because there are many factors that determine the calculated river length, such as the the position of the geographical source and the mouth, the scale of measurement, and the length measuring techniques (see also List of rivers by length).^[2]^[4]

^ ^a ^b "How Long Is the Amazon River?". Encyclopedia Britannica. Retrieved 2018-12-24.
^ ^a ^b ^c ^d Liu, Shaochuang; Lu, P; Liu, D; Jin, P; Wang, W (2009-03-01). "Pinpointing the sources and measuring the lengths of the principal rivers of the world". Int. J. Digital Earth. 2: 80–87. doi:10.1080/17538940902746082.
^ Dasgupta, Shreya. "Why the source of the Amazon river remains a mystery". www.bbc.com. Retrieved 2018-12-24.
^ ^a ^b "Where Does the Amazon River Begin?". National Geographic News. 2014-02-15. Retrieved 2018-12-25.
^ Society (U.S.), National Geographic (2014). Atlas of the World. National Geographic Books. ISBN 9781426213540.
^ "Land Steiermark - Geoinformationen (GIS Steiermark)". gis.steiermark.at. Retrieved 2019-05-14. {{cite web}}: Cite has empty unknown parameter: |dead-url= (help); no-break space character in |title= at position 35 (help)
^ ^a ^b Cite error: The named reference britannica-nile was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference britannica-amazon was invoked but never defined (see the help page).
^ Cite error: The named reference bbc-amazon was invoked but never defined (see the help page).
^ Roach, John. "Amazon Longer Than Nile River, Scientists Say". National Geographic. Retrieved 4 March 2015.
^ Cite error: The named reference inpe was invoked but never defined (see the help page).