Conversation so far:
what is the length of the longest river on earth
The Nile River is the longest river in the world, measuring at 4,130 mi.
what is the length of the second longest river on earth
The Amazon River is the second longest river in the world, measuring at 4,000 mi.
what is the source of the amazon river
The Mantaro, along with the Apurimac River, are the sources of the Amazon River, depending on the criteria used for definition.

Question: can you tell me the criteria
Self-contained rewrite: can you tell me the criteria used to define river length