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Both definitions of “source of the Amazon” advanced by its would-be finders are capricious. They imply that a river can be a lesser stream than its tributary, which runs counter to any plausible definition of tributary. Travel up the Amazon and at every fork take the branch with greater water flow. You will eventually reach the source of the Amazon. Take another branch, and you will reach the source of a tributary of the Amazon. Daniel Mathews
Portland, Ore. Geographer Andrew K. Johnston notes that there are several ways that the source of a river can be defined. He says his team chose the definition it did for the Amazon because it doesn’t depend on flow rates, which are extremely variable in mountain streams. Butch Kinerney of the U.S. Geological Survey says the source of a river has traditionally been defined according to water flow.–S. Perkins