Whether the Amazon or the Nile is the world's longest river is still disputed

A 2007 Brazilian-Peruvian expedition remeasured the Amazon's source and mouth using satellite data and argued it edges out the Nile as Earth's longest river. The dispute persists because there's no agreed definition of where a river's true source lies or which channel counts as its mouth, so different survey teams keep reaching different rankings using the same rivers.

— National Geographic News, Amazon Longer Than Nile River, Scientists Say — 2007